diff --git a/README.md b/README.md
index 60f250f..a4f0c34 100644
--- a/README.md
+++ b/README.md
@@ -40,7 +40,7 @@ fn main() {
   Operator precedence that does not bite (`x & 1 == 0` means what it says).
 - **Errors that talk like a person:**
   ```
-  game.fn:3:13: error: 'y' must be i32, but this is int (use i32(...) to convert)
+  game.fch:3:13: error: 'y' must be i32, but this is int (use i32(...) to convert)
       3 |     i32 y = x
         |             ^
   ```
@@ -60,9 +60,9 @@ nix-shell                           # or install LLVM 21 + libclang + cmake + ni
 cmake -S . -B build -G Ninja
 ninja -C build
 
-./build/finch run examples/hello.fn
-./build/finch build examples/tour.fn -o tour && ./tour
-tests/run.sh                        # 58 passed, 0 failed
+./build/finch run examples/hello.fch
+./build/finch build examples/tour.fch -o tour && ./tour
+tests/run.sh                        # 59 passed, 0 failed
 tests/boot.sh                       # the compiler written in Finch builds itself
 ```
 
@@ -97,16 +97,16 @@ Pick the guide that fits you:
 
 | File | Shows |
 |---|---|
-| [`examples/hello.fn`](examples/hello.fn) | the smallest program |
-| [`examples/tour.fn`](examples/tour.fn) | variables, functions, conditions, loops |
-| [`examples/guess.fn`](examples/guess.fn) | a guessing game: input, loops, C's `rand` |
-| [`examples/todo.fn`](examples/todo.fn) | structs, arrays, strings and files |
-| [`examples/structs.fn`](examples/structs.fn) | structs, arrays of structs, a linked list with `new`/`free` |
-| [`examples/types.fn`](examples/types.fn) | sized numbers and pointers |
-| [`examples/c_import.fn`](examples/c_import.fn) | `printf`, `math.h`, `malloc`/`free`, `stderr` |
-| [`examples/window.fn`](examples/window.fn) | a GLFW + OpenGL window |
-| [`examples/raylib.fn`](examples/raylib.fn) | raylib with C structs by value |
-| [`examples/llvm.fn`](examples/llvm.fn) | Finch building LLVM IR through the LLVM-C API |
+| [`examples/hello.fch`](examples/hello.fch) | the smallest program |
+| [`examples/tour.fch`](examples/tour.fch) | variables, functions, conditions, loops |
+| [`examples/guess.fch`](examples/guess.fch) | a guessing game: input, loops, C's `rand` |
+| [`examples/todo.fch`](examples/todo.fch) | structs, arrays, strings and files |
+| [`examples/structs.fch`](examples/structs.fch) | structs, arrays of structs, a linked list with `new`/`free` |
+| [`examples/types.fch`](examples/types.fch) | sized numbers and pointers |
+| [`examples/c_import.fch`](examples/c_import.fch) | `printf`, `math.h`, `malloc`/`free`, `stderr` |
+| [`examples/window.fch`](examples/window.fch) | a GLFW + OpenGL window |
+| [`examples/raylib.fch`](examples/raylib.fch) | raylib with C structs by value |
+| [`examples/llvm.fch`](examples/llvm.fch) | Finch building LLVM IR through the LLVM-C API |
 | [`boot/`](boot/) | the Finch compiler, written in Finch |
 
 ## Roadmap
diff --git a/README.pl.md b/README.pl.md
index 135b94c..c7ff4ba 100644
--- a/README.pl.md
+++ b/README.pl.md
@@ -40,7 +40,7 @@ fn main() {
   powodować niezdefiniowane zachowanie. Priorytety operatorów nie gryzą (`x & 1 == 0` znaczy to, co widać).
 - **Błędy mówią po ludzku:**
   ```
-  gra.fn:3:13: error: 'y' must be i32, but this is int (use i32(...) to convert)
+  gra.fch:3:13: error: 'y' must be i32, but this is int (use i32(...) to convert)
       3 |     i32 y = x
         |             ^
   ```
@@ -60,9 +60,9 @@ nix-shell                           # albo zainstaluj samodzielnie LLVM 21 + lib
 cmake -S . -B build -G Ninja
 ninja -C build
 
-./build/finch run examples/hello.fn
-./build/finch build examples/tour.fn -o tour && ./tour
-tests/run.sh                        # 58 passed, 0 failed
+./build/finch run examples/hello.fch
+./build/finch build examples/tour.fch -o tour && ./tour
+tests/run.sh                        # 59 passed, 0 failed
 tests/boot.sh                       # kompilator napisany w Finchu buduje sam siebie
 ```
 
@@ -97,16 +97,16 @@ Wybierz przewodnik dla siebie:
 
 | Plik | Pokazuje |
 |---|---|
-| [`examples/hello.fn`](examples/hello.fn) | najmniejszy program |
-| [`examples/tour.fn`](examples/tour.fn) | zmienne, funkcje, warunki, pętle |
-| [`examples/guess.fn`](examples/guess.fn) | gra w zgadywanie: wejście, pętle, `rand` z C |
-| [`examples/todo.fn`](examples/todo.fn) | struktury, tablice, teksty i pliki |
-| [`examples/structs.fn`](examples/structs.fn) | struktury, tablice struktur, lista wiązana z `new`/`free` |
-| [`examples/types.fn`](examples/types.fn) | liczby z rozmiarem i wskaźniki |
-| [`examples/c_import.fn`](examples/c_import.fn) | `printf`, `math.h`, `malloc`/`free`, `stderr` |
-| [`examples/window.fn`](examples/window.fn) | okno GLFW + OpenGL |
-| [`examples/raylib.fn`](examples/raylib.fn) | raylib ze strukturami C przez wartość |
-| [`examples/llvm.fn`](examples/llvm.fn) | Finch budujący LLVM IR przez API LLVM-C |
+| [`examples/hello.fch`](examples/hello.fch) | najmniejszy program |
+| [`examples/tour.fch`](examples/tour.fch) | zmienne, funkcje, warunki, pętle |
+| [`examples/guess.fch`](examples/guess.fch) | gra w zgadywanie: wejście, pętle, `rand` z C |
+| [`examples/todo.fch`](examples/todo.fch) | struktury, tablice, teksty i pliki |
+| [`examples/structs.fch`](examples/structs.fch) | struktury, tablice struktur, lista wiązana z `new`/`free` |
+| [`examples/types.fch`](examples/types.fch) | liczby z rozmiarem i wskaźniki |
+| [`examples/c_import.fch`](examples/c_import.fch) | `printf`, `math.h`, `malloc`/`free`, `stderr` |
+| [`examples/window.fch`](examples/window.fch) | okno GLFW + OpenGL |
+| [`examples/raylib.fch`](examples/raylib.fch) | raylib ze strukturami C przez wartość |
+| [`examples/llvm.fch`](examples/llvm.fch) | Finch budujący LLVM IR przez API LLVM-C |
 | [`boot/`](boot/) | kompilator Fincha napisany w Finchu |
 
 ## Plan rozwoju
diff --git a/boot/ast.fn b/boot/ast.fch
similarity index 100%
rename from boot/ast.fn
rename to boot/ast.fch
diff --git a/boot/gen.fn b/boot/gen.fch
similarity index 100%
rename from boot/gen.fn
rename to boot/gen.fch
diff --git a/boot/lexer.fn b/boot/lexer.fch
similarity index 100%
rename from boot/lexer.fn
rename to boot/lexer.fch
diff --git a/boot/main.fn b/boot/main.fch
similarity index 83%
rename from boot/main.fn
rename to boot/main.fch
index 7071413..0e985e6 100644
--- a/boot/main.fn
+++ b/boot/main.fch
@@ -1,7 +1,7 @@
 // finch-boot: the Finch compiler written in Finch.
 //
-//   finch-boot build <file.fn> [-o name]    compile a program
-//   finch-boot ir    <file.fn>              print the LLVM IR
+//   finch-boot build <file.fch> [-o name]    compile a program
+//   finch-boot ir    <file.fch>              print the LLVM IR
 //
 // It reads the program and the modules it imports, checks it, writes LLVM IR as text
 // and lets clang turn that into a program, linked with Finch's C runtime.
@@ -42,6 +42,11 @@ struct Loader {
 fn load(ptr[Loader] l, str path, str module) {
     if !file_exists(path) {
         if module.len > 0 {
+            old := path.sub(0, path.len - 4) + ".fn"
+            if file_exists(old) {
+                print("error: found " + old + ", but Finch files now end in .fch: rename it to " + path)
+                exit(1)
+            }
             print("error: can't find the module '" + module + "' (looked for " + path + ")")
         } else {
             print("error: can't open '" + path + "'")
@@ -59,15 +64,15 @@ fn load(ptr[Loader] l, str path, str module) {
     for name in imports {
         if !l.loaded.contains(name) {
             l.loaded.push(name)
-            load(l, dirOf(path) + "/" + name + ".fn", name)
+            load(l, dirOf(path) + "/" + name + ".fch", name)
         }
     }
 }
 
 fn usage() {
     print("finch-boot - the Finch compiler written in Finch")
-    print("usage: finch-boot build <file.fn> [-o name]")
-    print("       finch-boot ir <file.fn>")
+    print("usage: finch-boot build <file.fch> [-o name]")
+    print("       finch-boot ir <file.fch>")
     exit(1)
 }
 
diff --git a/boot/parser.fn b/boot/parser.fch
similarity index 100%
rename from boot/parser.fn
rename to boot/parser.fch
diff --git a/docs/en/for-beginners.md b/docs/en/for-beginners.md
index 6f17b69..f37b34b 100644
--- a/docs/en/for-beginners.md
+++ b/docs/en/for-beginners.md
@@ -92,11 +92,11 @@ fn main() {
 }
 ```
 
-3. Save it in the `Finch` folder as **`hello.fn`**. The `.fn` at the end tells everyone it is a Finch program.
+3. Save it in the `Finch` folder as **`hello.fch`**. The `.fch` at the end tells everyone it is a Finch program.
 4. In the Terminal, type:
 
 ```sh
-./build/finch run hello.fn
+./build/finch run hello.fch
 ```
 
 You will see:
@@ -652,14 +652,14 @@ fn main() {
 ```
 
 ```
-hello.fn:3:11: error: there is no variable named 'agee'
+hello.fch:3:11: error: there is no variable named 'agee'
     3 |     print(agee)
       |           ^
 ```
 
 How to read it:
 
-- `hello.fn` is the file,
+- `hello.fch` is the file,
 - `3` is the **line** number,
 - `11` is how far into the line (the **column**),
 - then the explanation, and the line itself with a `^` pointing at the problem.
@@ -816,7 +816,7 @@ fn main() {
 }
 ```
 
-`return` inside `main` ends the program. (A version with a random number is in `examples/guess.fn`.)
+`return` inside `main` ends the program. (A version with a random number is in `examples/guess.fch`.)
 </details>
 
 ### Project 7: Class average
diff --git a/docs/en/for-engineers.md b/docs/en/for-engineers.md
index 2e60de8..68bedff 100644
--- a/docs/en/for-engineers.md
+++ b/docs/en/for-engineers.md
@@ -48,7 +48,7 @@ Finch is a single-pass front end in front of LLVM: type checking and IR generati
 over the AST, with no intermediate representation of its own.
 
 ```
- main.fn ──► Loader::load() ── lex() ── parse() ──► Program  (+ each imported module, recursively)
+ main.fch ──► Loader::load() ── lex() ── parse() ──► Program  (+ each imported module, recursively)
                                                        │
                      all `import "x.h"` ──► importHeaders() (libclang) ──► CImports
                                                        │
@@ -100,7 +100,7 @@ Design decisions that shape the rest:
 | `src/abi.cpp` | 310 | System V x86-64 classification, C calls with structs by value, C-callable thunks |
 | `src/main.cpp` | 390 | Driver: CLI, loader, target machine, O2, object emission, runtime cache, linking, link-error advice, `run` |
 | `runtime/finch_rt.c` | 440 | The runtime library (§12) |
-| `boot/*.fn` | 3080 | The self-hosted compiler (§17) |
+| `boot/*.fch` | 3080 | The self-hosted compiler (§17) |
 
 ---
 
@@ -198,7 +198,7 @@ All nodes are in `src/ast.h`: small class hierarchies with an explicit `kind`, d
 | `Member` | `MemberExpr` | `obj`, `field` (`.x`, `.len`, `.ptr`, `.value`) |
 | `Index` | `IndexExpr` | `obj`, `index` |
 | `ArrayLit` | `ArrayLitExpr` | `elems` |
-| `Method` | `MethodExpr` | `obj`, `name`, `args`, `argNames`; also `module.fn(...)` |
+| `Method` | `MethodExpr` | `obj`, `name`, `args`, `argNames`; also `module.fch(...)` |
 
 | StmtKind | Node |
 |---|---|
@@ -236,7 +236,7 @@ Equality is structural; two `Struct`s are equal iff they share `info`.
 
 `Loader::load(path, module, from)` in `main.cpp` reads a file, appends it to `g_files` (positions carry
 the file index in `Pos::file`), lexes and parses it, then loads each `import name` that wasn't seen yet:
-`name.fn` next to the importing file, or in a `FINCH_PATH` folder. The result is a `std::vector<Program>`,
+`name.fch` next to the importing file, or in a `FINCH_PATH` folder. The result is a `std::vector<Program>`,
 main file first. Cycles are fine: a module is marked loaded before its own imports are followed.
 
 All files become **one LLVM module**. Each Finch module has a `ModuleScope` (functions, structs, the set
@@ -597,7 +597,7 @@ string building (`concat`, `from_int/uint/float/char/bool`, `sub`, `trim`, `uppe
 ### 13.1 Importing headers
 
 `importHeaders(imports, dirs)` parses each `import "x.h"` as its own translation unit from an in-memory
-`#include "x.h"`, with `-x c -std=gnu11`, `-I` for every `.fn` file's folder, and `-isystem` for each
+`#include "x.h"`, with `-x c -std=gnu11`, `-I` for every `.fch` file's folder, and `-isystem` for each
 directory the system C compiler searches (read from `$CC -E -v -x c /dev/null`, which is what makes it work
 on NixOS). `gnu11` because glibc hides `M_PI` and friends in strict ISO mode.
 
@@ -647,7 +647,7 @@ type and attributes, and rebuilds struct results the same way.
 struct parameters from registers or `byval` memory, calls the Finch function and lowers its result. Only
 C-compatible signatures are accepted.
 
-`tests/run/c_structs.fn` exercises `{float,float}`, `{float,float,float}`, `{u8×4}`, `{double,int}`,
+`tests/run/c_structs.fch` exercises `{float,float}`, `{float,float,float}`, `{u8×4}`, `{double,int}`,
 `{i64,i64}`, a 24-byte struct, a struct with a `char[8]`, nested structs, register exhaustion with 13
 struct arguments, and callbacks taking and returning structs, against a C library compiled with
 `link "abi.c"`. The results match the same calls made from C.
@@ -655,7 +655,7 @@ struct arguments, and callbacks taking and returning structs, against a C librar
 ### 13.4 Linking C code
 
 `link "name"` tries `pkg-config --libs name`, then `-lname`. `link "file.c"` compiles the file (relative
-to the `.fn` file) with `$CC -O2 -fPIC -c` into a temporary object; `.o` and `.a` files are passed
+to the `.fch` file) with `$CC -O2 -fPIC -c` into a temporary object; `.o` and `.a` files are passed
 through. When linking fails, `linkFailed()` maps undefined symbols back to the header that declared them
 and prints the `link` line to add.
 
@@ -666,7 +666,7 @@ and prints the `link` line to add.
 - **Target:** `sys::getDefaultTargetTriple()`, CPU `generic`, `Reloc::PIC_`. The data layout is set on the
   module before any IR is built (alignment of loads/stores depends on it).
 - **Optimization:** `PassBuilder::buildPerModuleDefaultPipeline(O2)`: clang's `-O2` middle end. `-O0`
-  skips it (`finch ir file.fn -O0` shows `Codegen`'s raw output).
+  skips it (`finch ir file.fch -O0` shows `Codegen`'s raw output).
 - **Emission:** legacy `PassManager` + `addPassesToEmitFile(ObjectFile)`.
 - **Linking:** `$CC prog.o rt.o [user objects] -lm [libs]`. On Nix the cc wrapper turns `-L` paths into
   `RPATH`, so programs run without `LD_LIBRARY_PATH`.
@@ -707,11 +707,11 @@ there's an obvious fix, how to fix it. There are no warnings. An LLVM verifier f
 
 | File | Contents |
 |---|---|
-| `boot/lexer.fn` | the lexer (tokens are `{kind, text, line, col, nl}`; kinds are words, keywords and operators are their own kind) |
-| `boot/ast.fn` | `Type` (`kind`, `elem` as a 0/1-element array, `name`, `module`), one generic `Node` for everything, `Program` |
-| `boot/parser.fn` | the same grammar and newline rules as the C++ parser, precedence climbing for binary operators |
-| `boot/gen.fn` | type checker + **textual LLVM IR** generator: scopes, places, coercions, copies, helpers, printing, runtime checks, array/str methods |
-| `boot/main.fn` | loader for modules and the driver: writes `.ll`, runs `clang -O2 file.ll runtime/finch_rt.c` |
+| `boot/lexer.fch` | the lexer (tokens are `{kind, text, line, col, nl}`; kinds are words, keywords and operators are their own kind) |
+| `boot/ast.fch` | `Type` (`kind`, `elem` as a 0/1-element array, `name`, `module`), one generic `Node` for everything, `Program` |
+| `boot/parser.fch` | the same grammar and newline rules as the C++ parser, precedence climbing for binary operators |
+| `boot/gen.fch` | type checker + **textual LLVM IR** generator: scopes, places, coercions, copies, helpers, printing, runtime checks, array/str methods |
+| `boot/main.fch` | loader for modules and the driver: writes `.ll`, runs `clang -O2 file.ll runtime/finch_rt.c` |
 
 It supports the core language: `int float bool char str`, `[]T`, structs (defaults, named/positional
 constructors), `ptr[T]`/`addr`/`new`/`free`/`null`, all statements except `defer`, modules, and the
@@ -753,9 +753,9 @@ with the expected one. It takes about 2.3 s for the self-hosted compiler to comp
 
 ## 19. Testing
 
-- `tests/run.sh`: every `tests/run/*.fn` with `fn main` must print exactly its `.out` (stdin from `.in`
-  if present); files without `main` are modules or helpers. Every `tests/fail/*.fn` must fail with the text
-  from its `// expect:` line. Currently **58 passed, 0 failed**.
+- `tests/run.sh`: every `tests/run/*.fch` with `fn main` must print exactly its `.out` (stdin from `.in`
+  if present); files without `main` are modules or helpers. Every `tests/fail/*.fch` must fail with the text
+  from its `// expect:` line. Currently **59 passed, 0 failed**.
 - `MEMCHECK=1 tests/run.sh`: the same, plus valgrind on every program (no leaks, no invalid access).
 - `tests/boot.sh`: the self-hosting fixpoint and the subset run (§17).
 
@@ -787,7 +787,7 @@ with the expected one. It takes about 2.3 s for the self-hosted compiler to comp
 
 | Function | Does |
 |---|---|
-| `Loader::load` / `findModule` | read, lex, parse a file and recursively its modules / locate `name.fn` |
+| `Loader::load` / `findModule` | read, lex, parse a file and recursively its modules / locate `name.fch` |
 | `hostMachine`, `optimize`, `emitObject` | target machine, O2 pipeline, object file |
 | `runtimeObject` | compile and cache the runtime |
 | `capture`, `libFlags`, `link` | run a command / pkg-config or `-l` / the link step, compiling `link "x.c"` files |
@@ -858,7 +858,7 @@ with the expected one. It takes about 2.3 s for the self-hosted compiler to comp
 
 **A built-in function:** add the name to `isBuiltin()`, handle it in `call()`, implement it (in IR, or as
 a runtime function declared in `rt()`'s table), add `tests/run/` and `tests/fail/` cases. If it should
-exist in the self-hosted compiler too, add it to `boot/gen.fn`'s `call()` and `runtimeDecls()`.
+exist in the self-hosted compiler too, add it to `boot/gen.fch`'s `call()` and `runtimeDecls()`.
 
 **An array or str method:** add a branch in `arrayMethod()` or the str part of `method()`; if it changes
 the array, add its name to the `changing` sets in `method()` and `mutatesExpr()`, so borrowing stays correct.
diff --git a/docs/en/for-technicians.md b/docs/en/for-technicians.md
index 282b752..98ce4cc 100644
--- a/docs/en/for-technicians.md
+++ b/docs/en/for-technicians.md
@@ -38,7 +38,7 @@ compiler works inside. For that, see the [guide for engineers](for-engineers.md)
 
 ## 1. What Finch is
 
-Finch is a **compiled** programming language. The `finch` tool translates your `.fn`
+Finch is a **compiled** programming language. The `finch` tool translates your `.fch`
 file into a real program (a native executable, like the ones made from C), using the
 **LLVM** compiler toolkit, the same backend that powers clang, Rust and Swift.
 
@@ -106,9 +106,9 @@ Finch's small runtime library, which is kept in `~/.cache/finch`. Set `CC` to us
 ## 3. The finch command
 
 ```
-finch run   <file.fn> [args...]    compile and run right away
-finch build <file.fn> [-o name]    compile into a program (default name: the file's name)
-finch ir    <file.fn>              print the LLVM IR (for the curious)
+finch run   <file.fch> [args...]    compile and run right away
+finch build <file.fch> [-o name]    compile into a program (default name: the file's name)
+finch ir    <file.fch>              print the LLVM IR (for the curious)
 finch version                      show the version
 
 options:
@@ -120,10 +120,10 @@ options:
 Examples:
 
 ```sh
-finch run hello.fn                 # prints the output
-finch run tool.fn input.txt -v     # arguments after the file go to the program
-finch build game.fn -o mygame      # creates ./mygame
-finch build game.fn -g -O0         # a build for the debugger
+finch run hello.fch                 # prints the output
+finch run tool.fch input.txt -v     # arguments after the file go to the program
+finch build game.fch -o mygame      # creates ./mygame
+finch build game.fch -g -O0         # a build for the debugger
 ```
 
 `finch run` passes on your program's exit code. If the program crashes, it says how
@@ -139,7 +139,7 @@ finch build game.fn -g -O0         # a build for the debugger
    several lines */
 
 import "math.h"          // C headers (optional), at the top
-import shapes            // Finch modules (optional): shapes.fn next to this file
+import shapes            // Finch modules (optional): shapes.fch next to this file
 link "m"                 // C libraries or C files to link (optional)
 
 struct Point {           // your own types
@@ -502,11 +502,11 @@ Using `.value`, `.field` or `[i]` on a `null` pointer **stops the program** with
 
 ## 16. Modules
 
-Split a program into files. `import shapes` loads `shapes.fn` from the same folder (or from a
+Split a program into files. `import shapes` loads `shapes.fch` from the same folder (or from a
 folder listed in the `FINCH_PATH` environment variable, separated by `:`).
 
 ```c
-// shapes.fn
+// shapes.fch
 struct Box {
     float w
     float h
@@ -518,7 +518,7 @@ fn area(Box b) -> float {
 ```
 
 ```c
-// main.fn
+// main.fch
 import shapes
 
 fn main() {
@@ -540,7 +540,7 @@ fn main() {
 ```c
 import "stdio.h"
 import "GLFW/glfw3.h"     // a path inside the system include folders
-import "mylib.h"          // a header next to your .fn file
+import "mylib.h"          // a header next to your .fch file
 ```
 
 Finch reads the header with **libclang** (clang's C parser) and makes these usable:
@@ -554,7 +554,7 @@ Finch reads the header with **libclang** (clang's C parser) and makes these usab
 
 ```c
 link "glfw"              // uses libglfw.so (pkg-config is asked first, if installed)
-link "helpers.c"         // compile and link your own C file (next to the .fn file)
+link "helpers.c"         // compile and link your own C file (next to the .fch file)
 link "prebuilt.a"        // or an object file / static library
 ```
 
@@ -617,7 +617,7 @@ Its parameters must be C types (numbers, pointers, C structs).
 ### Compile errors
 
 ```
-game.fn:12:9: error: 'score' must be int, but this is str
+game.fch:12:9: error: 'score' must be int, but this is str
    12 |     score = "high"
       |             ^
 ```
@@ -643,9 +643,9 @@ Instead of undefined behavior (as in C), Finch stops with a message and exit cod
 ## 19. Debugging
 
 ```sh
-finch build game.fn -g -O0 -o game
+finch build game.fch -g -O0 -o game
 gdb ./game
-(gdb) break game.fn:12
+(gdb) break game.fch:12
 (gdb) run
 (gdb) bt                # where are we, through which functions
 (gdb) info locals       # all variables
@@ -665,7 +665,7 @@ visible; without it the optimizer may remove some.
 | `… come from "x.h", but its library isn't linked` | Add `link "name"` at the top of the file. |
 | `the library 'x' wasn't found` | Not installed, or a wrong name. On Nix, add it to `shell.nix` and run inside `nix-shell`. |
 | `the C function 'f' can't be used from Finch yet` | It uses a union, bit fields or another unsupported type by value. Look for a variant that takes pointers. |
-| `can't find the module 'x'` | Put `x.fn` next to the importing file, or set `FINCH_PATH`. |
+| `can't find the module 'x'` | Put `x.fch` next to the importing file, or set `FINCH_PATH`. |
 | `couldn't build the Finch runtime` | No working C compiler. Install gcc or clang, or set `CC`. |
 | A library is not found when the program starts | It was linked from a folder the system doesn't search. Run inside the same `nix-shell`, or install it system-wide. |
 | `clang` inside `nix-shell` can't find `stdio.h` | You have an old `shell.nix`: `llvmPackages.clang` must be listed before `llvmPackages.libclang`. |
@@ -692,7 +692,7 @@ Finch/
 ```
 
 ```sh
-tests/run.sh                 # → 58 passed, 0 failed
+tests/run.sh                 # → 59 passed, 0 failed
 MEMCHECK=1 tests/run.sh      # the same under valgrind: no leaks, no bad memory access
 tests/boot.sh                # the self-hosting check (needs clang)
 ```
diff --git a/docs/pl/dla-inzynierow.md b/docs/pl/dla-inzynierow.md
index e71d9b1..79d97dc 100644
--- a/docs/pl/dla-inzynierow.md
+++ b/docs/pl/dla-inzynierow.md
@@ -48,7 +48,7 @@ Finch to jednoprzebiegowy front-end przed LLVM: sprawdzanie typów i generowanie
 po AST, bez własnej reprezentacji pośredniej.
 
 ```
- main.fn ──► Loader::load() ── lex() ── parse() ──► Program  (+ każdy importowany moduł, rekurencyjnie)
+ main.fch ──► Loader::load() ── lex() ── parse() ──► Program  (+ każdy importowany moduł, rekurencyjnie)
                                                        │
                 wszystkie `import "x.h"` ──► importHeaders() (libclang) ──► CImports
                                                        │
@@ -100,7 +100,7 @@ Decyzje, które kształtują resztę:
 | `src/abi.cpp` | 310 | Klasyfikacja System V x86-64, wywołania C ze strukturami przez wartość, wrappery wywoływalne z C |
 | `src/main.cpp` | 390 | Sterownik: CLI, ładowanie, maszyna docelowa, O2, emisja, cache runtime'u, linkowanie, porady przy błędach linkera, `run` |
 | `runtime/finch_rt.c` | 440 | Biblioteka uruchomieniowa (§12) |
-| `boot/*.fn` | 3080 | Kompilator samohostujący (§17) |
+| `boot/*.fch` | 3080 | Kompilator samohostujący (§17) |
 
 ---
 
@@ -199,7 +199,7 @@ Wszystkie węzły są w `src/ast.h`: małe hierarchie klas z jawnym `kind`, rozg
 | `Member` | `MemberExpr` | `obj`, `field` (`.x`, `.len`, `.ptr`, `.value`) |
 | `Index` | `IndexExpr` | `obj`, `index` |
 | `ArrayLit` | `ArrayLitExpr` | `elems` |
-| `Method` | `MethodExpr` | `obj`, `name`, `args`, `argNames`; także `modul.fn(...)` |
+| `Method` | `MethodExpr` | `obj`, `name`, `args`, `argNames`; także `modul.fch(...)` |
 
 | StmtKind | Węzeł |
 |---|---|
@@ -236,7 +236,7 @@ Równość jest strukturalna; dwa `Struct` są równe, gdy mają to samo `info`.
 
 `Loader::load(path, module, from)` w `main.cpp` czyta plik, dopisuje go do `g_files` (pozycje niosą
 indeks pliku w `Pos::file`), leksuje go i parsuje, a potem wczytuje każdy jeszcze niewidziany `import nazwa`:
-`nazwa.fn` obok importującego pliku albo w folderze z `FINCH_PATH`. Wynik to `std::vector<Program>`,
+`nazwa.fch` obok importującego pliku albo w folderze z `FINCH_PATH`. Wynik to `std::vector<Program>`,
 z głównym plikiem na początku. Cykle nie przeszkadzają: moduł jest oznaczany jako wczytany, zanim
 zaczniemy podążać za jego importami.
 
@@ -605,7 +605,7 @@ własność (`copy`, `own`, `drop`, `from_c`), tablice (`reserve`, `make`, `resi
 ### 13.1 Import nagłówków
 
 `importHeaders(imports, dirs)` parsuje każdy `import "x.h"` jako osobną jednostkę translacji z pliku
-w pamięci `#include "x.h"`, z `-x c -std=gnu11`, `-I` dla folderu każdego pliku `.fn` i `-isystem` dla
+w pamięci `#include "x.h"`, z `-x c -std=gnu11`, `-I` dla folderu każdego pliku `.fch` i `-isystem` dla
 każdego katalogu przeszukiwanego przez systemowy kompilator C (odczytanego z `$CC -E -v -x c /dev/null`,
 dzięki czemu działa to na NixOS). `gnu11`, bo glibc w trybie ścisłego ISO ukrywa `M_PI` i podobne.
 
@@ -657,7 +657,7 @@ i atrybutami, a wyniki-struktury odbudowuje w ten sam sposób.
 `cThunk(fn)` to odwrotność dla `addr(fn)`: wewnętrzna funkcja z sygnaturą C, która składa parametry-struktury
 z rejestrów albo pamięci `byval`, woła funkcję Fincha i obniża jej wynik. Akceptowane są tylko sygnatury zgodne z C.
 
-`tests/run/c_structs.fn` sprawdza `{float,float}`, `{float,float,float}`, `{u8×4}`, `{double,int}`,
+`tests/run/c_structs.fch` sprawdza `{float,float}`, `{float,float,float}`, `{u8×4}`, `{double,int}`,
 `{i64,i64}`, strukturę 24-bajtową, strukturę z `char[8]`, zagnieżdżone struktury, wyczerpanie rejestrów
 przy 13 argumentach-strukturach oraz wywołania zwrotne przyjmujące i zwracające struktury, na bibliotece C
 kompilowanej przez `link "abi.c"`. Wyniki zgadzają się z tymi samymi wywołaniami zrobionymi z C.
@@ -665,7 +665,7 @@ kompilowanej przez `link "abi.c"`. Wyniki zgadzają się z tymi samymi wywołani
 ### 13.4 Linkowanie kodu C
 
 `link "nazwa"` próbuje `pkg-config --libs nazwa`, a potem `-lnazwa`. `link "plik.c"` kompiluje plik
-(względem pliku `.fn`) przez `$CC -O2 -fPIC -c` do tymczasowego obiektu; pliki `.o` i `.a` przechodzą
+(względem pliku `.fch`) przez `$CC -O2 -fPIC -c` do tymczasowego obiektu; pliki `.o` i `.a` przechodzą
 bez zmian. Gdy linkowanie się nie uda, `linkFailed()` mapuje niezdefiniowane symbole na nagłówek, który
 je zadeklarował, i wypisuje linijkę `link` do dopisania.
 
@@ -676,7 +676,7 @@ je zadeklarował, i wypisuje linijkę `link` do dopisania.
 - **Cel:** `sys::getDefaultTargetTriple()`, CPU `generic`, `Reloc::PIC_`. Data layout jest ustawiany na
   module, zanim powstanie jakikolwiek IR (od niego zależy wyrównanie loadów i store'ów).
 - **Optymalizacja:** `PassBuilder::buildPerModuleDefaultPipeline(O2)`: środkowa część `-O2` z clanga.
-  `-O0` ją pomija (`finch ir plik.fn -O0` pokazuje surowe wyjście `Codegen`).
+  `-O0` ją pomija (`finch ir plik.fch -O0` pokazuje surowe wyjście `Codegen`).
 - **Emisja:** stary `PassManager` + `addPassesToEmitFile(ObjectFile)`.
 - **Linkowanie:** `$CC prog.o rt.o [obiekty użytkownika] -lm [biblioteki]`. Na Niksie wrapper cc zamienia
   ścieżki `-L` na `RPATH`, więc programy działają bez `LD_LIBRARY_PATH`.
@@ -717,11 +717,11 @@ wypisuje `internal compiler error (please report)` i kończy proces z kodem 2.
 
 | Plik | Zawartość |
 |---|---|
-| `boot/lexer.fn` | lekser (tokeny to `{kind, text, line, col, nl}`; rodzaje to słowa, a słowa kluczowe i operatory są swoim własnym rodzajem) |
-| `boot/ast.fn` | `Type` (`kind`, `elem` jako tablica 0/1 elementów, `name`, `module`), jeden ogólny `Node` na wszystko, `Program` |
-| `boot/parser.fn` | ta sama gramatyka i reguły nowych linii co parser C++, wspinanie się po priorytetach dla operatorów binarnych |
-| `boot/gen.fn` | sprawdzanie typów + generator **tekstowego LLVM IR**: zasięgi, miejsca, konwersje, kopie, helpery, wypisywanie, kontrole, metody tablic i tekstów |
-| `boot/main.fn` | ładowanie modułów i sterownik: zapisuje `.ll`, uruchamia `clang -O2 plik.ll runtime/finch_rt.c` |
+| `boot/lexer.fch` | lekser (tokeny to `{kind, text, line, col, nl}`; rodzaje to słowa, a słowa kluczowe i operatory są swoim własnym rodzajem) |
+| `boot/ast.fch` | `Type` (`kind`, `elem` jako tablica 0/1 elementów, `name`, `module`), jeden ogólny `Node` na wszystko, `Program` |
+| `boot/parser.fch` | ta sama gramatyka i reguły nowych linii co parser C++, wspinanie się po priorytetach dla operatorów binarnych |
+| `boot/gen.fch` | sprawdzanie typów + generator **tekstowego LLVM IR**: zasięgi, miejsca, konwersje, kopie, helpery, wypisywanie, kontrole, metody tablic i tekstów |
+| `boot/main.fch` | ładowanie modułów i sterownik: zapisuje `.ll`, uruchamia `clang -O2 plik.ll runtime/finch_rt.c` |
 
 Obsługuje rdzeń języka: `int float bool char str`, `[]T`, struktury (wartości domyślne, konstruktory po
 nazwie i po kolei), `ptr[T]`/`addr`/`new`/`free`/`null`, wszystkie instrukcje poza `defer`, moduły oraz
@@ -763,9 +763,9 @@ wynik z oczekiwanym. Kompilacja samego siebie zajmuje kompilatorowi samohostują
 
 ## 19. Testy
 
-- `tests/run.sh`: każdy `tests/run/*.fn` z `fn main` musi wypisać dokładnie swój `.out` (stdin z `.in`, jeśli
-  jest); pliki bez `main` to moduły albo pliki pomocnicze. Każdy `tests/fail/*.fn` musi się nie udać z tekstem
-  z linijki `// expect:`. Obecnie **58 passed, 0 failed**.
+- `tests/run.sh`: każdy `tests/run/*.fch` z `fn main` musi wypisać dokładnie swój `.out` (stdin z `.in`, jeśli
+  jest); pliki bez `main` to moduły albo pliki pomocnicze. Każdy `tests/fail/*.fch` musi się nie udać z tekstem
+  z linijki `// expect:`. Obecnie **59 passed, 0 failed**.
 - `MEMCHECK=1 tests/run.sh`: to samo plus valgrind na każdym programie (bez wycieków i złych dostępów).
 - `tests/boot.sh`: punkt stały samohostowania i przebieg na podzbiorze (§17).
 
@@ -797,7 +797,7 @@ wynik z oczekiwanym. Kompilacja samego siebie zajmuje kompilatorowi samohostują
 
 | Funkcja | Robi |
 |---|---|
-| `Loader::load` / `findModule` | czyta, leksuje i parsuje plik oraz rekurencyjnie jego moduły / szuka `nazwa.fn` |
+| `Loader::load` / `findModule` | czyta, leksuje i parsuje plik oraz rekurencyjnie jego moduły / szuka `nazwa.fch` |
 | `hostMachine`, `optimize`, `emitObject` | maszyna docelowa, potok O2, plik obiektowy |
 | `runtimeObject` | kompiluje runtime i trzyma go w cache |
 | `capture`, `libFlags`, `link` | uruchamia polecenie / pkg-config albo `-l` / linkowanie, z kompilacją plików z `link "x.c"` |
@@ -868,7 +868,7 @@ wynik z oczekiwanym. Kompilacja samego siebie zajmuje kompilatorowi samohostują
 
 **Funkcja wbudowana:** dodaj nazwę do `isBuiltin()`, obsłuż ją w `call()`, zaimplementuj (w IR albo jako
 funkcję runtime'u zadeklarowaną w tabeli `rt()`), dodaj przypadki w `tests/run/` i `tests/fail/`. Jeśli ma
-istnieć też w kompilatorze samohostującym, dodaj ją do `call()` i `runtimeDecls()` w `boot/gen.fn`.
+istnieć też w kompilatorze samohostującym, dodaj ją do `call()` i `runtimeDecls()` w `boot/gen.fch`.
 
 **Metoda tablicy albo tekstu:** dodaj gałąź w `arrayMethod()` albo w części `str` w `method()`; jeśli
 zmienia tablicę, dopisz jej nazwę do zbiorów `changing` w `method()` i `mutatesExpr()`, żeby pożyczanie
diff --git a/docs/pl/dla-poczatkujacych.md b/docs/pl/dla-poczatkujacych.md
index 4ff632e..7422f78 100644
--- a/docs/pl/dla-poczatkujacych.md
+++ b/docs/pl/dla-poczatkujacych.md
@@ -92,11 +92,11 @@ fn main() {
 }
 ```
 
-3. Zapisz plik w folderze `Finch` pod nazwą **`czesc.fn`**. Końcówka `.fn` mówi, że to program w Finchu.
+3. Zapisz plik w folderze `Finch` pod nazwą **`czesc.fch`**. Końcówka `.fch` mówi, że to program w Finchu.
 4. W Terminalu wpisz:
 
 ```sh
-./build/finch run czesc.fn
+./build/finch run czesc.fch
 ```
 
 Zobaczysz:
@@ -659,14 +659,14 @@ fn main() {
 ```
 
 ```
-czesc.fn:3:11: error: there is no variable named 'wiekk'
+czesc.fch:3:11: error: there is no variable named 'wiekk'
     3 |     print(wiekk)
       |           ^
 ```
 
 Jak to czytać:
 
-- `czesc.fn` to plik,
+- `czesc.fch` to plik,
 - `3` to numer **linijki**,
 - `11` to który znak w linijce (**kolumna**),
 - potem wyjaśnienie, a pod nim ta linijka ze strzałką `^` wskazującą problem.
@@ -823,7 +823,7 @@ fn main() {
 }
 ```
 
-`return` w `main` kończy program. (Wersja z losową liczbą jest w `examples/guess.fn`.)
+`return` w `main` kończy program. (Wersja z losową liczbą jest w `examples/guess.fch`.)
 </details>
 
 ### Projekt 7: Średnia ocen
diff --git a/docs/pl/dla-technikow.md b/docs/pl/dla-technikow.md
index e638005..1ee3e5e 100644
--- a/docs/pl/dla-technikow.md
+++ b/docs/pl/dla-technikow.md
@@ -38,7 +38,7 @@ kompilator działa w środku. Do tego służy [przewodnik dla inżynierów](dla-
 
 ## 1. Czym jest Finch
 
-Finch to język **kompilowany**. Narzędzie `finch` tłumaczy plik `.fn` na prawdziwy program
+Finch to język **kompilowany**. Narzędzie `finch` tłumaczy plik `.fch` na prawdziwy program
 (natywny plik wykonywalny, taki jak te z C). Korzysta przy tym z **LLVM**, tego samego zaplecza
 kompilatorów, na którym działają clang, Rust i Swift.
 
@@ -107,9 +107,9 @@ Inny kompilator wskażesz zmienną `CC`.
 ## 3. Polecenie finch
 
 ```
-finch run   <plik.fn> [argumenty...]   skompiluj i od razu uruchom
-finch build <plik.fn> [-o nazwa]       skompiluj do programu (domyślna nazwa: nazwa pliku)
-finch ir    <plik.fn>                  pokaż LLVM IR (dla ciekawskich)
+finch run   <plik.fch> [argumenty...]   skompiluj i od razu uruchom
+finch build <plik.fch> [-o nazwa]       skompiluj do programu (domyślna nazwa: nazwa pliku)
+finch ir    <plik.fch>                  pokaż LLVM IR (dla ciekawskich)
 finch version                          pokaż wersję
 
 opcje:
@@ -121,10 +121,10 @@ opcje:
 Przykłady:
 
 ```sh
-finch run hello.fn                 # wypisuje wynik
-finch run narzedzie.fn dane.txt -v # argumenty po nazwie pliku trafiają do programu
-finch build gra.fn -o mojagra      # tworzy ./mojagra
-finch build gra.fn -g -O0          # wersja dla debuggera
+finch run hello.fch                 # wypisuje wynik
+finch run narzedzie.fch dane.txt -v # argumenty po nazwie pliku trafiają do programu
+finch build gra.fch -o mojagra      # tworzy ./mojagra
+finch build gra.fch -g -O0          # wersja dla debuggera
 ```
 
 `finch run` przekazuje dalej kod wyjścia programu. Jeśli program się wysypie, powie jak
@@ -140,7 +140,7 @@ finch build gra.fn -g -O0          # wersja dla debuggera
    kilka linijek */
 
 import "math.h"          // nagłówki C (opcjonalne), na górze
-import ksztalty          // moduły Fincha (opcjonalne): ksztalty.fn obok tego pliku
+import ksztalty          // moduły Fincha (opcjonalne): ksztalty.fch obok tego pliku
 link "m"                 // biblioteki albo pliki C do dolinkowania (opcjonalne)
 
 struct Punkt {           // własne typy
@@ -503,11 +503,11 @@ Użycie `.value`, `.pole` albo `[i]` na wskaźniku `null` **zatrzymuje program**
 
 ## 16. Moduły
 
-Program możesz podzielić na pliki. `import ksztalty` wczytuje `ksztalty.fn` z tego samego folderu
+Program możesz podzielić na pliki. `import ksztalty` wczytuje `ksztalty.fch` z tego samego folderu
 (albo z folderu wymienionego w zmiennej środowiskowej `FINCH_PATH`, rozdzielonych `:`).
 
 ```c
-// ksztalty.fn
+// ksztalty.fch
 struct Pudlo {
     float w
     float h
@@ -519,7 +519,7 @@ fn pole(Pudlo p) -> float {
 ```
 
 ```c
-// main.fn
+// main.fch
 import ksztalty
 
 fn main() {
@@ -541,7 +541,7 @@ fn main() {
 ```c
 import "stdio.h"
 import "GLFW/glfw3.h"     // ścieżka w systemowych katalogach nagłówków
-import "mojalib.h"        // nagłówek obok twojego pliku .fn
+import "mojalib.h"        // nagłówek obok twojego pliku .fch
 ```
 
 Finch czyta nagłówek przez **libclang** (parser C z clanga) i udostępnia:
@@ -555,7 +555,7 @@ Finch czyta nagłówek przez **libclang** (parser C z clanga) i udostępnia:
 
 ```c
 link "glfw"              // używa libglfw.so (najpierw pyta pkg-config, jeśli jest)
-link "pomocnicze.c"      // skompiluj i dołącz własny plik C (obok pliku .fn)
+link "pomocnicze.c"      // skompiluj i dołącz własny plik C (obok pliku .fch)
 link "gotowe.a"          // albo plik obiektowy / bibliotekę statyczną
 ```
 
@@ -618,7 +618,7 @@ Jej parametry muszą być typami C (liczby, wskaźniki, struktury C).
 ### Błędy kompilacji
 
 ```
-gra.fn:12:9: error: 'wynik' must be int, but this is str
+gra.fch:12:9: error: 'wynik' must be int, but this is str
    12 |     wynik = "wysoki"
       |             ^
 ```
@@ -645,9 +645,9 @@ Zamiast niezdefiniowanego zachowania (jak w C) Finch zatrzymuje się z komunikat
 ## 19. Debugowanie
 
 ```sh
-finch build gra.fn -g -O0 -o gra
+finch build gra.fch -g -O0 -o gra
 gdb ./gra
-(gdb) break gra.fn:12
+(gdb) break gra.fch:12
 (gdb) run
 (gdb) bt                # gdzie jesteśmy, przez które funkcje
 (gdb) info locals       # wszystkie zmienne
@@ -667,7 +667,7 @@ zmienne widoczne; bez niego optymalizator może część z nich usunąć.
 | `… come from "x.h", but its library isn't linked` | Dopisz `link "nazwa"` na górze pliku. |
 | `the library 'x' wasn't found` | Nie jest zainstalowana albo nazwa jest zła. Na Niksie dopisz ją do `shell.nix` i pracuj w `nix-shell`. |
 | `the C function 'f' can't be used from Finch yet` | Używa unii, pól bitowych albo innego nieobsługiwanego typu przez wartość. Poszukaj wariantu ze wskaźnikami. |
-| `can't find the module 'x'` | Połóż `x.fn` obok importującego pliku albo ustaw `FINCH_PATH`. |
+| `can't find the module 'x'` | Połóż `x.fch` obok importującego pliku albo ustaw `FINCH_PATH`. |
 | `couldn't build the Finch runtime` | Brak działającego kompilatora C. Zainstaluj gcc lub clang albo ustaw `CC`. |
 | Biblioteka nie znajduje się przy starcie programu | Zlinkowano ją z folderu, którego system nie przeszukuje. Uruchamiaj w tym samym `nix-shell` albo zainstaluj ją systemowo. |
 | `clang` w `nix-shell` nie widzi `stdio.h` | Masz stary `shell.nix`: `llvmPackages.clang` musi być przed `llvmPackages.libclang`. |
@@ -694,7 +694,7 @@ Finch/
 ```
 
 ```sh
-tests/run.sh                 # → 58 passed, 0 failed
+tests/run.sh                 # → 59 passed, 0 failed
 MEMCHECK=1 tests/run.sh      # to samo pod valgrindem: bez wycieków i złych dostępów do pamięci
 tests/boot.sh                # sprawdzenie samohostowania (wymaga clanga)
 ```
diff --git a/examples/c_import.fn b/examples/c_import.fch
similarity index 100%
rename from examples/c_import.fn
rename to examples/c_import.fch
diff --git a/examples/guess.fn b/examples/guess.fch
similarity index 100%
rename from examples/guess.fn
rename to examples/guess.fch
diff --git a/examples/hello.fn b/examples/hello.fch
similarity index 100%
rename from examples/hello.fn
rename to examples/hello.fch
diff --git a/examples/llvm.fn b/examples/llvm.fch
similarity index 100%
rename from examples/llvm.fn
rename to examples/llvm.fch
diff --git a/examples/raylib.fn b/examples/raylib.fch
similarity index 100%
rename from examples/raylib.fn
rename to examples/raylib.fch
diff --git a/examples/structs.fn b/examples/structs.fch
similarity index 100%
rename from examples/structs.fn
rename to examples/structs.fch
diff --git a/examples/todo.fn b/examples/todo.fch
similarity index 100%
rename from examples/todo.fn
rename to examples/todo.fch
diff --git a/examples/tour.fn b/examples/tour.fch
similarity index 100%
rename from examples/tour.fn
rename to examples/tour.fch
diff --git a/examples/types.fn b/examples/types.fch
similarity index 100%
rename from examples/types.fn
rename to examples/types.fch
diff --git a/examples/window.fn b/examples/window.fch
similarity index 100%
rename from examples/window.fn
rename to examples/window.fch
diff --git a/shell.nix b/shell.nix
index 7087ecf..3f44c9e 100644
--- a/shell.nix
+++ b/shell.nix
@@ -10,7 +10,7 @@ pkgs.mkShell {
     ninja
     pkg-config              # finds the right flags for `link "..."`
 
-    # for examples/window.fn and examples/raylib.fn
+    # for examples/window.fch and examples/raylib.fch
     glfw
     libGL
     raylib
diff --git a/src/ast.h b/src/ast.h
index 6f72215..2cb02fb 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -298,10 +298,10 @@ struct Import {
 
 struct Link {
     Pos pos;
-    std::string lib;  // "glfw" -> libglfw.so; "helpers.c" / ".o" / ".a" -> a file next to the .fn file
+    std::string lib;  // "glfw" -> libglfw.so; "helpers.c" / ".o" / ".a" -> a file next to the .fch file
 };
 
-// One .fn file.
+// One .fch file.
 struct Program {
     std::string module;  // "" for the main file, else the module's name
     std::string path;
diff --git a/src/cimport.h b/src/cimport.h
index ecc3605..fe4be0d 100644
--- a/src/cimport.h
+++ b/src/cimport.h
@@ -52,5 +52,5 @@ struct CImports {
     std::map<std::string, CStruct> structs;
 };
 
-// `dirs`: folders searched first for "local.h" headers (the .fn files' folders).
+// `dirs`: folders searched first for "local.h" headers (the .fch files' folders).
 CImports importHeaders(const std::vector<Import> &imports, const std::vector<std::string> &dirs);
diff --git a/src/main.cpp b/src/main.cpp
index d120afa..e755330 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -34,9 +34,9 @@ static void usage() {
     std::fprintf(stderr,
                  "Finch " FINCH_VERSION " - small, quick, sharp.\n\n"
                  "usage:\n"
-                 "  finch run   <file.fn> [args...]    compile and run\n"
-                 "  finch build <file.fn> [-o name]    compile to a program\n"
-                 "  finch ir    <file.fn>              show the generated LLVM IR\n"
+                 "  finch run   <file.fch> [args...]    compile and run\n"
+                 "  finch build <file.fch> [-o name]    compile to a program\n"
+                 "  finch ir    <file.fch>              show the generated LLVM IR\n"
                  "  finch version                      show the version\n"
                  "\n"
                  "  -l <lib>   link a C library, same as  link \"lib\"  in the file\n"
@@ -68,7 +68,12 @@ struct Loader {
     void load(const std::string &path, const std::string &module, Pos from) {
         std::string text;
         if (!readFile(path, text)) {
-            if (from.line) failAt(from.file, from.line, from.col, "can't find the module '" + module + "' (looked for " + path + ")");
+            if (from.line) {
+                std::string old = path.substr(0, path.size() - 4) + ".fn";
+                if (sys::fs::exists(old))
+                    failAt(from.file, from.line, from.col, "found " + old + ", but Finch files now end in .fch: rename it to " + path);
+                failAt(from.file, from.line, from.col, "can't find the module '" + module + "' (looked for " + path + ")");
+            }
             std::fprintf(stderr, "error: can't open '%s'\n", path.c_str());
             std::exit(1);
         }
@@ -91,12 +96,12 @@ struct Loader {
 
     // next to the importing file, then in $FINCH_PATH folders
     static std::string findModule(const std::string &dir, const std::string &name) {
-        std::string here = dir + "/" + name + ".fn";
+        std::string here = dir + "/" + name + ".fch";
         if (sys::fs::exists(here)) return here;
         if (const char *fp = std::getenv("FINCH_PATH")) {
             std::stringstream ss(fp);
             for (std::string d; std::getline(ss, d, ':');)
-                if (!d.empty() && sys::fs::exists(d + "/" + name + ".fn")) return d + "/" + name + ".fn";
+                if (!d.empty() && sys::fs::exists(d + "/" + name + ".fch")) return d + "/" + name + ".fch";
         }
         return here;
     }
diff --git a/src/parser.cpp b/src/parser.cpp
index be451f7..14889fa 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -137,7 +137,7 @@ private:
         expect(Tok::Link);
         l.lib = expect(Tok::String, "a library name in quotes, like link \"glfw\"").text;
         auto ends = [&](const char *x) { size_t n = std::strlen(x); return l.lib.size() > n && l.lib.compare(l.lib.size() - n, n, x) == 0; };
-        if (ends(".c") || ends(".o") || ends(".a")) return l;  // your own C code, next to the .fn file
+        if (ends(".c") || ends(".o") || ends(".a")) return l;  // your own C code, next to the .fch file
         if (l.lib.rfind("lib", 0) == 0 || l.lib.find(".so") != std::string::npos || l.lib.find('/') != std::string::npos)
             fail(l.pos.line, l.pos.col, "write just the library's name: for libglfw.so that's  link \"glfw\"");
         return l;
diff --git a/tests/boot.sh b/tests/boot.sh
index 1d1305e..ef4748e 100755
--- a/tests/boot.sh
+++ b/tests/boot.sh
@@ -7,10 +7,10 @@ cd "$(dirname "$0")/.."
 tmp=$(mktemp -d)
 trap 'rm -rf "$tmp"' EXIT
 
-build/finch build boot/main.fn -o "$tmp/boot1" || exit 1
-"$tmp/boot1" build boot/main.fn -o "$tmp/boot2" || exit 1
-"$tmp/boot1" ir boot/main.fn > "$tmp/stage2.ll"
-"$tmp/boot2" ir boot/main.fn > "$tmp/stage3.ll"
+build/finch build boot/main.fch -o "$tmp/boot1" || exit 1
+"$tmp/boot1" build boot/main.fch -o "$tmp/boot2" || exit 1
+"$tmp/boot1" ir boot/main.fch > "$tmp/stage2.ll"
+"$tmp/boot2" ir boot/main.fch > "$tmp/stage3.ll"
 if cmp -s "$tmp/stage2.ll" "$tmp/stage3.ll"; then
     echo "bootstrap: stage 2 and stage 3 are identical ($(wc -l < "$tmp/stage3.ll") lines of IR)"
 else
@@ -19,17 +19,17 @@ else
 fi
 
 pass=0; failed=0; skipped=0
-for f in tests/run/*.fn; do
+for f in tests/run/*.fch; do
     grep -q "^fn main" "$f" || continue
     # the bootstrap compiler doesn't do C imports, sized numbers or defer
     # (checked in the test and in the modules it imports)
     files="$f"
-    for m in $(sed -n 's/^import \([a-z_]*\)$/\1/p' "$f"); do files="$files tests/run/$m.fn"; done
+    for m in $(sed -n 's/^import \([a-z_]*\)$/\1/p' "$f"); do files="$files tests/run/$m.fch"; done
     if grep -qE '^import "|^link |\b(u8|u16|u32|u64|i8|i16|i32|f32|f64)\b|defer |0x' $files; then
         skipped=$((skipped + 1))
         continue
     fi
-    input="${f%.fn}.in"
+    input="${f%.fch}.in"
     [ -f "$input" ] || input=/dev/null
     if ! "$tmp/boot2" build "$f" -o "$tmp/prog" > "$tmp/err" 2>&1; then
         failed=$((failed + 1))
@@ -38,12 +38,12 @@ for f in tests/run/*.fn; do
         continue
     fi
     got=$("$tmp/prog" < "$input" 2>&1)
-    if [ "$got" == "$(cat "${f%.fn}.out")" ]; then
+    if [ "$got" == "$(cat "${f%.fch}.out")" ]; then
         pass=$((pass + 1))
     else
         failed=$((failed + 1))
         echo "FAIL (output) $f"
-        diff <(echo "$got") "${f%.fn}.out" | head -10
+        diff <(echo "$got") "${f%.fch}.out" | head -10
     fi
 done
 echo "self-hosted compiler: $pass passed, $failed failed, $skipped skipped (features it doesn't have)"
diff --git a/tests/fail/array_compare.fn b/tests/fail/array_compare.fch
similarity index 100%
rename from tests/fail/array_compare.fn
rename to tests/fail/array_compare.fch
diff --git a/tests/fail/bad_int.fn b/tests/fail/bad_int.fch
similarity index 100%
rename from tests/fail/bad_int.fn
rename to tests/fail/bad_int.fch
diff --git a/tests/fail/bad_link_name.fn b/tests/fail/bad_link_name.fch
similarity index 100%
rename from tests/fail/bad_link_name.fn
rename to tests/fail/bad_link_name.fch
diff --git a/tests/fail/bool_bitand.fn b/tests/fail/bool_bitand.fch
similarity index 100%
rename from tests/fail/bool_bitand.fn
rename to tests/fail/bool_bitand.fch
diff --git a/tests/fail/condition_not_bool.fn b/tests/fail/condition_not_bool.fch
similarity index 100%
rename from tests/fail/condition_not_bool.fn
rename to tests/fail/condition_not_bool.fch
diff --git a/tests/fail/defer_return.fn b/tests/fail/defer_return.fch
similarity index 100%
rename from tests/fail/defer_return.fn
rename to tests/fail/defer_return.fch
diff --git a/tests/fail/div_zero_const.fn b/tests/fail/div_zero_const.fch
similarity index 100%
rename from tests/fail/div_zero_const.fn
rename to tests/fail/div_zero_const.fch
diff --git a/tests/fail/div_zero_runtime.fn b/tests/fail/div_zero_runtime.fch
similarity index 100%
rename from tests/fail/div_zero_runtime.fn
rename to tests/fail/div_zero_runtime.fch
diff --git a/tests/fail/each_readonly.fn b/tests/fail/each_readonly.fch
similarity index 100%
rename from tests/fail/each_readonly.fn
rename to tests/fail/each_readonly.fch
diff --git a/tests/fail/empty_array.fn b/tests/fail/empty_array.fch
similarity index 100%
rename from tests/fail/empty_array.fn
rename to tests/fail/empty_array.fch
diff --git a/tests/fail/float_to_int.fn b/tests/fail/float_to_int.fch
similarity index 100%
rename from tests/fail/float_to_int.fn
rename to tests/fail/float_to_int.fch
diff --git a/tests/fail/forgot_import.fn b/tests/fail/forgot_import.fch
similarity index 100%
rename from tests/fail/forgot_import.fn
rename to tests/fail/forgot_import.fch
diff --git a/tests/fail/free_value.fn b/tests/fail/free_value.fch
similarity index 100%
rename from tests/fail/free_value.fn
rename to tests/fail/free_value.fch
diff --git a/tests/fail/index_range.fn b/tests/fail/index_range.fch
similarity index 100%
rename from tests/fail/index_range.fn
rename to tests/fail/index_range.fch
diff --git a/tests/fail/lib_hint.fn b/tests/fail/lib_hint.fch
similarity index 100%
rename from tests/fail/lib_hint.fn
rename to tests/fail/lib_hint.fch
diff --git a/tests/fail/literal_too_big.fn b/tests/fail/literal_too_big.fch
similarity index 100%
rename from tests/fail/literal_too_big.fn
rename to tests/fail/literal_too_big.fch
diff --git a/tests/fail/loop_var_readonly.fn b/tests/fail/loop_var_readonly.fch
similarity index 100%
rename from tests/fail/loop_var_readonly.fn
rename to tests/fail/loop_var_readonly.fch
diff --git a/tests/fail/missing_header.fn b/tests/fail/missing_header.fch
similarity index 100%
rename from tests/fail/missing_header.fn
rename to tests/fail/missing_header.fch
diff --git a/tests/fail/missing_lib.fn b/tests/fail/missing_lib.fch
similarity index 100%
rename from tests/fail/missing_lib.fn
rename to tests/fail/missing_lib.fch
diff --git a/tests/fail/missing_return.fn b/tests/fail/missing_return.fch
similarity index 100%
rename from tests/fail/missing_return.fn
rename to tests/fail/missing_return.fch
diff --git a/tests/fail/mixed_signs.fn b/tests/fail/mixed_signs.fch
similarity index 100%
rename from tests/fail/mixed_signs.fn
rename to tests/fail/mixed_signs.fch
diff --git a/tests/fail/module_missing.fn b/tests/fail/module_missing.fch
similarity index 100%
rename from tests/fail/module_missing.fn
rename to tests/fail/module_missing.fch
diff --git a/tests/fail/module_not_imported.fn b/tests/fail/module_not_imported.fch
similarity index 100%
rename from tests/fail/module_not_imported.fn
rename to tests/fail/module_not_imported.fch
diff --git a/tests/fail/no_main.fn b/tests/fail/no_main.fch
similarity index 100%
rename from tests/fail/no_main.fn
rename to tests/fail/no_main.fch
diff --git a/tests/fail/non_ascii_name.fn b/tests/fail/non_ascii_name.fch
similarity index 100%
rename from tests/fail/non_ascii_name.fn
rename to tests/fail/non_ascii_name.fch
diff --git a/tests/fail/null_value.fn b/tests/fail/null_value.fch
similarity index 100%
rename from tests/fail/null_value.fn
rename to tests/fail/null_value.fch
diff --git a/tests/fail/old_extension.fch b/tests/fail/old_extension.fch
new file mode 100644
index 0000000..2764d5c
--- /dev/null
+++ b/tests/fail/old_extension.fch
@@ -0,0 +1,3 @@
+// expect: but Finch files now end in .fch
+import oldmodule
+fn main() {}
diff --git a/tests/fail/old_function_syntax.fn b/tests/fail/old_function_syntax.fch
similarity index 100%
rename from tests/fail/old_function_syntax.fn
rename to tests/fail/old_function_syntax.fch
diff --git a/tests/fail/oldmodule.fn b/tests/fail/oldmodule.fn
new file mode 100644
index 0000000..768ca7c
--- /dev/null
+++ b/tests/fail/oldmodule.fn
@@ -0,0 +1,2 @@
+fn hi() {
+}
diff --git a/tests/fail/pop_empty.fn b/tests/fail/pop_empty.fch
similarity index 100%
rename from tests/fail/pop_empty.fn
rename to tests/fail/pop_empty.fch
diff --git a/tests/fail/push_type.fn b/tests/fail/push_type.fch
similarity index 100%
rename from tests/fail/push_type.fn
rename to tests/fail/push_type.fch
diff --git a/tests/fail/redeclare.fn b/tests/fail/redeclare.fch
similarity index 100%
rename from tests/fail/redeclare.fn
rename to tests/fail/redeclare.fch
diff --git a/tests/fail/str_null.fn b/tests/fail/str_null.fch
similarity index 100%
rename from tests/fail/str_null.fn
rename to tests/fail/str_null.fch
diff --git a/tests/fail/str_plus_int.fn b/tests/fail/str_plus_int.fch
similarity index 100%
rename from tests/fail/str_plus_int.fn
rename to tests/fail/str_plus_int.fch
diff --git a/tests/fail/struct_count.fn b/tests/fail/struct_count.fch
similarity index 100%
rename from tests/fail/struct_count.fn
rename to tests/fail/struct_count.fch
diff --git a/tests/fail/struct_no_field.fn b/tests/fail/struct_no_field.fch
similarity index 100%
rename from tests/fail/struct_no_field.fn
rename to tests/fail/struct_no_field.fch
diff --git a/tests/fail/struct_self.fn b/tests/fail/struct_self.fch
similarity index 100%
rename from tests/fail/struct_self.fn
rename to tests/fail/struct_self.fch
diff --git a/tests/fail/two_statements.fn b/tests/fail/two_statements.fch
similarity index 100%
rename from tests/fail/two_statements.fn
rename to tests/fail/two_statements.fch
diff --git a/tests/fail/type_mismatch.fn b/tests/fail/type_mismatch.fch
similarity index 100%
rename from tests/fail/type_mismatch.fn
rename to tests/fail/type_mismatch.fch
diff --git a/tests/fail/unclosed_brace.fn b/tests/fail/unclosed_brace.fch
similarity index 100%
rename from tests/fail/unclosed_brace.fn
rename to tests/fail/unclosed_brace.fch
diff --git a/tests/fail/undefined_var.fn b/tests/fail/undefined_var.fch
similarity index 100%
rename from tests/fail/undefined_var.fn
rename to tests/fail/undefined_var.fch
diff --git a/tests/fail/unreachable.fn b/tests/fail/unreachable.fch
similarity index 100%
rename from tests/fail/unreachable.fn
rename to tests/fail/unreachable.fch
diff --git a/tests/fail/untyped_null.fn b/tests/fail/untyped_null.fch
similarity index 100%
rename from tests/fail/untyped_null.fn
rename to tests/fail/untyped_null.fch
diff --git a/tests/fail/untyped_ptr.fn b/tests/fail/untyped_ptr.fch
similarity index 100%
rename from tests/fail/untyped_ptr.fn
rename to tests/fail/untyped_ptr.fch
diff --git a/tests/run.sh b/tests/run.sh
index 28f0c88..c85f2b8 100755
--- a/tests/run.sh
+++ b/tests/run.sh
@@ -1,9 +1,9 @@
 #!/usr/bin/env bash
 # Finch test suite.
-#   tests/run/*.fn   must compile and print exactly what is in the matching .out file
+#   tests/run/*.fch   must compile and print exactly what is in the matching .out file
 #                    (files without `fn main` are modules or helpers and are skipped;
 #                     a matching .in file is fed to the program as its input)
-#   tests/fail/*.fn  must fail; the first line is "// expect: <text that must appear in the error>"
+#   tests/fail/*.fch  must fail; the first line is "// expect: <text that must appear in the error>"
 #   MEMCHECK=1 tests/run.sh   also runs every program under valgrind: no leaks, no bad memory access
 set -u
 cd "$(dirname "$0")"
@@ -12,10 +12,10 @@ pass=0; failed=0
 tmp=$(mktemp -d)
 trap 'rm -rf "$tmp"' EXIT
 
-for f in run/*.fn; do
+for f in run/*.fch; do
     grep -q "^fn main" "$f" || continue
-    want="${f%.fn}.out"
-    input="${f%.fn}.in"
+    want="${f%.fch}.out"
+    input="${f%.fch}.in"
     [ -f "$input" ] || input=/dev/null
     got=$("$FINCH" run "$f" < "$input" 2>&1)
     if [ "$got" != "$(cat "$want")" ]; then
@@ -25,7 +25,7 @@ for f in run/*.fn; do
         continue
     fi
     if [ "${MEMCHECK:-0}" = 1 ]; then
-        exe="$tmp/$(basename "${f%.fn}")"
+        exe="$tmp/$(basename "${f%.fch}")"
         "$FINCH" build "$f" -o "$exe" >/dev/null 2>&1
         if ! valgrind -q --leak-check=full --errors-for-leak-kinds=all --error-exitcode=99 "$exe" < "$input" >/dev/null 2>"$tmp/vg"; then
             failed=$((failed + 1))
@@ -37,7 +37,8 @@ for f in run/*.fn; do
     pass=$((pass + 1))
 done
 
-for f in fail/*.fn; do
+for f in fail/*.fch; do
+    grep -q "^// expect:" "$f" || continue
     expect=$(head -1 "$f" | sed -n 's|^// expect: ||p')
     got=$("$FINCH" run "$f" < /dev/null 2>&1)
     status=$?
diff --git a/tests/run/basics.fn b/tests/run/basics.fch
similarity index 100%
rename from tests/run/basics.fn
rename to tests/run/basics.fch
diff --git a/tests/run/bits.fn b/tests/run/bits.fch
similarity index 100%
rename from tests/run/bits.fn
rename to tests/run/bits.fch
diff --git a/tests/run/c_structs.fn b/tests/run/c_structs.fch
similarity index 100%
rename from tests/run/c_structs.fn
rename to tests/run/c_structs.fch
diff --git a/tests/run/cimport.fn b/tests/run/cimport.fch
similarity index 100%
rename from tests/run/cimport.fn
rename to tests/run/cimport.fch
diff --git a/tests/run/control.fn b/tests/run/control.fch
similarity index 100%
rename from tests/run/control.fn
rename to tests/run/control.fch
diff --git a/tests/run/defer.fn b/tests/run/defer.fch
similarity index 100%
rename from tests/run/defer.fn
rename to tests/run/defer.fch
diff --git a/tests/run/exitcode.fn b/tests/run/exitcode.fch
similarity index 100%
rename from tests/run/exitcode.fn
rename to tests/run/exitcode.fch
diff --git a/tests/run/files.fn b/tests/run/files.fch
similarity index 100%
rename from tests/run/files.fn
rename to tests/run/files.fch
diff --git a/tests/run/functions.fn b/tests/run/functions.fch
similarity index 100%
rename from tests/run/functions.fn
rename to tests/run/functions.fch
diff --git a/tests/run/geometry.fn b/tests/run/geometry.fch
similarity index 100%
rename from tests/run/geometry.fn
rename to tests/run/geometry.fch
diff --git a/tests/run/input.fn b/tests/run/input.fch
similarity index 100%
rename from tests/run/input.fn
rename to tests/run/input.fch
diff --git a/tests/run/memory.fn b/tests/run/memory.fch
similarity index 100%
rename from tests/run/memory.fn
rename to tests/run/memory.fch
diff --git a/tests/run/modules.fn b/tests/run/modules.fch
similarity index 100%
rename from tests/run/modules.fn
rename to tests/run/modules.fch
diff --git a/tests/run/pointers.fn b/tests/run/pointers.fch
similarity index 100%
rename from tests/run/pointers.fn
rename to tests/run/pointers.fch
diff --git a/tests/run/ptr_value_field.fn b/tests/run/ptr_value_field.fch
similarity index 100%
rename from tests/run/ptr_value_field.fn
rename to tests/run/ptr_value_field.fch
diff --git a/tests/run/shapes.fn b/tests/run/shapes.fch
similarity index 100%
rename from tests/run/shapes.fn
rename to tests/run/shapes.fch
diff --git a/tests/run/structs_arrays_strings.fn b/tests/run/structs_arrays_strings.fch
similarity index 100%
rename from tests/run/structs_arrays_strings.fn
rename to tests/run/structs_arrays_strings.fch
diff --git a/tests/run/types.fn b/tests/run/types.fch
similarity index 100%
rename from tests/run/types.fn
rename to tests/run/types.fch