Finch/docs/en/for-technicians.md

Finch for Technicians

For people who know their way around a computer, but little or no programming. This guide covers installing Finch, using the finch tool, the whole language, memory, modules, C libraries, debugging and fixing common problems. It does not cover how the compiler works inside. For that, see the guide for engineers.

🇵🇱 Polska wersja: dla-technikow.md

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Contents

1. What Finch is
2. Installing
3. The finch command
4. How a Finch file looks
5. Variables
6. Types
7. Operators
8. Control flow
9. Functions
10. Arrays
11. Text (str)
12. Structs
13. Memory: who frees what
14. Pointers
15. Built-in functions
16. Modules
17. Using C libraries
18. Errors
19. Debugging
20. Troubleshooting
21. Project layout and tests
22. Current limits

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1. What Finch is

Finch is a compiled programming language. The finch tool translates your .fn 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.

What that gives you:

• Speed close to C. No interpreter, virtual machine or garbage collector. Array bounds checks and other safety checks cost almost nothing after optimization.
• Small, standalone programs that you can copy and run.
• Memory handled for you. Lists and text are freed automatically at the end of the block that owns them, with no garbage collector involved.
• Direct access to C libraries. import "stdio.h" and call anything in it, even functions that take structs.

The design rules: one way to do each thing, C-like syntax without C's traps, and error messages that tell you how to fix the problem.

---

2. Installing

Finch currently runs on Linux, x86-64. It has been tested with LLVM 21. Other LLVM versions will likely fail to compile, because LLVM's C++ API changes between releases.

Option A: Nix (recommended, nothing to install by hand)

git clone https://github.com/Nexoniarz/Finch.git
cd Finch
nix-shell                        # downloads LLVM, libclang, clang, cmake, ninja, pkg-config
cmake -S . -B build -G Ninja
ninja -C build
./build/finch version            # finch 2.0.0 (LLVM 21.x)

Option B: your distribution's packages

You need: a C/C++ compiler, CMake ≥ 3.20, Ninja (or Make), LLVM 21 development files, libclang 21 development files, and optionally pkg-config.

Distribution	Packages (names can differ slightly by version)
Debian / Ubuntu	build-essential cmake ninja-build pkg-config llvm-21-dev libclang-21-dev
Fedora	gcc-c++ cmake ninja-build pkgconf llvm-devel clang-devel
Arch	base-devel cmake ninja pkgconf llvm clang

These have not been tested; if CMake cannot find LLVM, point it there:

cmake -S . -B build -G Ninja -DLLVM_DIR=/usr/lib/llvm-21/lib/cmake/llvm
ninja -C build

Putting finch on your PATH (optional)

sudo cp build/finch /usr/local/bin/
finch version

At run time Finch needs a C compiler (cc). It is used to link programs, and once to build Finch's small runtime library, which is kept in ~/.cache/finch. Set CC to use a different one.

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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 version                      show the version

options:
  -l <lib>   link a C library, same as  link "lib"  in the file
  -g         add debug info (for gdb / lldb)
  -O0        skip optimizations

Examples:

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 passes on your program's exit code. If the program crashes, it says how (for example the program crashed: Segmentation fault).

---

4. How a Finch file looks

// comments start with two slashes
/* or span
   several lines */

import "math.h"          // C headers (optional), at the top
import shapes            // Finch modules (optional): shapes.fn next to this file
link "m"                 // C libraries or C files to link (optional)

struct Point {           // your own types
    int x
    int y
}

fn main() {              // the program starts here
    print("Hello")
}

Rules worth knowing:

• One statement per line. There are no semicolons. To split a long expression, break the line after an operator (a +⏎ b) or anywhere inside brackets ( ) or [ ].
• Curly braces are always required after if, else, for, while, and the opening { goes on the same line.
• No brackets around conditions: if x > 5 {, not if (x > 5) {.
• Functions and structs can be in any order in the file.
• main is fn main(), fn main() -> int (returns the exit code) or fn main([]str args) (gets the command-line arguments; args[0] is the program's own path).

---

5. Variables

x := 5               // new variable, type inferred (int)
int y = 10           // new variable, type written explicitly
int z                // no value: starts at zero ("" for str, [] for arrays, null for pointers)
Point p              // a struct with no value: zero, or the struct's default values

x = 7                // assign
x += 1               // also -= *= /= %=

• A name can be declared once in a function. Finch does not allow reusing a name in an inner block (no "shadowing"), because that is a common source of confusion.
• Variables exist from their declaration to the end of the { } block they are in.
• Names: ASCII letters, digits and _, not starting with a digit. Non-ASCII letters are allowed only inside strings. Type names (int, u8, …), built-in function names and keywords are reserved.

Keywords: fn return if else while for in break continue true false null import link struct defer

---

6. Types

Everyday types

Type	Holds	Example
int	whole number, 64-bit (same as i64)	42, -7, 0xFF, 1_000_000
float	decimal number, 64-bit (same as f64)	3.14
bool	true or false
char	one byte-sized character	'A', '\n'
str	text	"hello\tworld"
[]T	a list (array) of T	[1, 2, 3]
your structs	named groups of values	Point(1, 2)

String and char escapes: \n new line, \t tab, \r, \0, \\, \", \'.

Sized types (for files, hardware, C libraries)

Signed	Range	Unsigned	Range
i8	−128 … 127	u8	0 … 255
i16	−32 768 … 32 767	u16	0 … 65 535
i32	about ±2.1 billion	u32	0 … about 4.29 billion
i64	about ±9.2 × 10¹⁸	u64	0 … about 1.8 × 10¹⁹

f32 is a 32-bit decimal number (about 7 significant digits), f64 a 64-bit one (about 15–16).

Conversions

Finch converts automatically only when nothing can be lost:

From	To	Automatic?
a smaller whole number	a bigger one of the same signedness (i8→i32, u8→u64)	✅
unsigned	a strictly bigger signed (u8→i16, u32→i64)	✅
any whole number	float / f32	✅
f32	f64	✅
null	any pointer	✅
any pointer	ptr, and ptr to any pointer	✅
anything else		❌ write it: type(value)

Number literals adapt. A number written in the code fits into any type that can hold it: u8 b = 200 works, u8 b = 300 is an error. The same applies to constants imported from C headers.

Explicit conversions use the type name like a function:

int(3.99)        // 3     (cuts off the fraction)
float(7) / 2     // 3.5
u8(300)          // 44    (keeps the lowest 8 bits)
int('A')         // 65
char(66)         // 'B'
bool(0)          // false (any non-zero number is true)
str(42)          // "42"  (also floats, bools, chars; str([]u8) turns bytes into text)
int("42")        // 42    (stops the program if the text isn't a number)
float("2.5")     // 2.5
ptr(p)           // a typed pointer as an untyped one
str(p)           // a C char pointer as text

Fixed-size numbers wrap around, like real hardware: a u8 holding 255 plus 1 becomes 0. Calculations happen in the type of the values involved, so u8(250) + 10 is 4, not 260.

---

7. Operators

By priority (higher binds tighter)

Priority	Operators	Meaning
5	* / % << >> &	multiply, divide, remainder, shift left/right, bitwise AND
4	+ - | ^	add (and join text), subtract, bitwise OR, bitwise XOR
3	== != < <= > >=	comparisons
2	&&	logical AND
1	||	logical OR

Unary (in front of a value): -x negate, !x logical NOT, ~x flip all bits.

Different from C, on purpose: bit operators bind tighter than comparisons, so flags & 4 != 0 means (flags & 4) != 0.

Notes:

• / on whole numbers rounds toward zero: 7 / 2 is 3. % takes the sign of the left side.
• Dividing by zero stops the program with runtime error: division by zero and the line number.
• && and || only evaluate the right side if needed.
• Text: + joins, == != compare contents, < > compare alphabetically (byte by byte).
• Pointers can be compared with == and !=, including against null. A str is never null.
• Arrays and structs can't be compared with ==; compare their parts.

---

8. Control flow

if x > 10 {
    ...
} else if x > 0 {
    ...
} else {
    ...
}

while count > 0 {
    count -= 1
}

for i in 0..10 {          // i = 0, 1, …, 9  (the end is not included)
    if i == 3 { continue }
    if i == 8 { break }
}

for name in names {       // every element of an array (or every char of a str)
    print(name)
}

• A condition must be a bool. if x { with a number is an error; write if x != 0 {.
• In for i in a..b, both ends are evaluated once. i is an int and cannot be changed in the loop.
• In for x in list, x cannot be changed either. To change elements, loop over the indexes: for i in 0..list.len { list[i] = ... }. Adding to the list inside the loop is fine.
• while true { ... } loops until a break or return.
• Code after return, break or continue in the same block is an error, because it can never run.

defer

defer runs a statement when the current block ends, however it ends: normally, by return, break or continue. Several defers run in reverse order. Handy for cleanup:

f := fopen("data.txt", "r")
defer fclose(f)
// ... use f; it is closed when the block ends, on every path

A deferred statement can't contain return, break or continue.

---

9. Functions

fn name(type1 param1, type2 param2) -> return_type {
    ...
    return value
}

fn no_result(str message) {       // no "->": returns nothing
    print(message)
}

• Arguments behave like copies: changing a parameter never changes the caller's variable. (Finch only really copies when needed: a function that doesn't change an array gets it for free.) To change the caller's variable, pass a pointer: see Pointers.
• A function with -> type must return a value on every path. Finch checks this.
• A function cannot be defined inside another function.

---

10. Arrays

nums := [5, 3, 8]          // an array of int
[]str names                // an empty array of str
grid := [[1, 2], [3, 4]]   // arrays of arrays
[]f32 xs = [1, 2.5]        // the type decides what the numbers become

Operation	Meaning
a.len	number of elements
a[i]	element i (from 0); out of range stops the program with a clear message
a[i] = x	change an element
a.push(x)	add at the end
a.pop()	remove the last one and give it back
a.insert(i, x)	put x at position i, moving the rest
a.remove(i)	remove position i and give it back
a.clear()	remove everything
a.resize(n)	make it n long (new elements are zero)
a.contains(x), a.find(x)	is x there? its index, or -1 (numbers, chars, bools, str)
a.sort(), a.reverse()	in place (numbers, chars, str)
a.slice(s, e)	a new array with elements s … e-1
a.join(sep)	for []str: one text with sep between the parts
a.ptr	the address of the first element (for C)
print(a)	prints [1, 2, 3]

Assigning copies: after b := a, changing b does not change a.

---

11. Text (str)

s := "Hello"
s += ", world"            // join
print(s.len, s[0])        // 12 H   (.len counts bytes; ą, ż, … take 2)
s[0] = 'J'                // change a character

Method	Gives back
s.sub(a, b)	the part from a to b-1
s.find(t)	where t starts, or -1
s.contains(t), s.starts_with(t), s.ends_with(t)	bool
s.split(sep)	[]str; split("") gives single characters
s.trim()	without spaces/newlines at both ends
s.upper(), s.lower()	A–Z / a–z changed (ASCII letters)
s.replace(a, b)	every a replaced by b
s.repeat(n)	s n times
s.bytes()	[]u8 with the bytes
s.ptr	a ptr[char] for C (valid while s lives)

str keeps its length, so .len is instant, and it is always NUL-terminated, so passing it to C as char* works.

---

12. Structs

struct Player {
    str name
    int lives = 3            // a default value
    []int scores
    Point pos                // structs inside structs
}

p := Player(name: "Ola")                     // by name: missing fields get their default (or zero)
q := Point(1, 2)                             // in order: then every field must be given
p.lives -= 1
p.scores.push(10)
p.pos.x = 5
print(p)       // Player(name: "Ola", lives: 2, scores: [10], pos: Point(x: 5, y: 0))

• Assigning or passing a struct copies it, together with its arrays and text.
• A struct can't contain itself directly; use ptr[Node] or []Node for that field (trees, lists).
• Structs have no methods. Write functions that take the struct: fn heal(Player p) -> Player.

---

13. Memory: who frees what

You never free arrays, text or structs. Finch does it for you, at a predictable moment: when the block that owns the value ends (}), or earlier on return, break, continue.

• A variable owns its value. Assigning copies it, so two variables never share one list.
• A value produced by a function call or an expression is moved where it's stored: no copy.
• return list moves the variable out of the function: no copy.
• Function parameters are lent: the function gets the caller's value without a copy. If the function changes the parameter, Finch makes it a private copy first. You never notice except in speed.

For memory you manage yourself (linked lists, trees, sharing between structures):

node := new(Node(value: 1))     // put a value on the heap, get a ptr[Node]
defer free(node)                // free(...) gives it back (and frees its arrays and text)

Memory from C (malloc, library objects) is freed with C's functions, as in C.

---

14. Pointers

A pointer holds the address of a value. Finch uses words instead of C's * and &:

C	Finch	Meaning
int *p	ptr[int] p	p points to an int
&x	addr(x)	the address of x (also addr(a[i]), addr(s.field))
*p	p.value	the value p points to
p->field	p.field	a field of the struct p points to
p[i]	p[i]	C-style indexing (no bounds check)
NULL	null	points nowhere
void *	ptr	points to something of unknown type

fn double(ptr[int] p) {
    p.value = p.value * 2
}

x := 21
double(addr(x))           // x is 42

If a struct has its own field called value, then p.value on a pointer to it means that field.

Using .value, .field or [i] on a null pointer stops the program with a clear message.

⚠️ A pointer to a variable must not be used after the variable's block has ended. Finch does not check this.

---

15. Built-in functions

Function	What it does
print(a, b, ...)	Prints all values separated by spaces, then a new line. Works with every type, including arrays and structs.
input(), input("question")	Reads one line typed by the user (without the newline). Empty text at end of input.
read_file(path)	The whole file as str. Stops the program with a clear message if it can't be read.
write_file(path, text)	Writes (replaces) the file; gives back true if it worked.
file_exists(path)	true / false
shell(command)	Runs a shell command, gives back its exit code.
exit(code)	Ends the program right away.
addr(x)	A pointer to x; addr(function) gives C a function pointer (callback).
new(value), free(p)	Manual heap memory (see Memory).
int(x), str(x), u8(x), …	Conversions (see Conversions).

---

16. Modules

Split a program into files. import shapes loads shapes.fn from the same folder (or from a folder listed in the FINCH_PATH environment variable, separated by :).

// shapes.fn
struct Box {
    float w
    float h
}

fn area(Box b) -> float {
    return b.w * b.h
}

// main.fn
import shapes

fn main() {
    b := shapes.Box(2, 3)
    print(shapes.area(b))
    shapes.Box other         // a module's type in a declaration
}

• Everything in a module is used with the module's name in front: shapes.area, shapes.Box.
• A module doesn't have main. Modules can import other modules.

---

17. Using C libraries

Step 1: import the header

import "stdio.h"
import "GLFW/glfw3.h"     // a path inside the system include folders
import "mylib.h"          // a header next to your .fn file

Finch reads the header with libclang (clang's C parser) and makes these usable:

• functions, including ones with a variable number of arguments, like printf,
• structs, by value and by pointer (Vector2, SDL_Rect, CXCursor, …), with their fields,
• enum values and simple #define constants (M_PI, GL_COLOR_BUFFER_BIT),
• global variables, like stdout and stderr.

Step 2: link the library

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 "prebuilt.a"        // or an object file / static library

If you forget the link line, Finch tells you exactly what is missing:

error: glfwInit, glfwCreateWindow, glfwMakeContextCurrent and 5 more come from "GLFW/glfw3.h", but its library isn't linked
  ...
      link "glfw"

How C types appear in Finch

C	Finch
char, signed char, unsigned char	char, i8, u8
short, int, long, long long	i16, i32, i64, i64 (by actual size)
unsigned …	u16, u32, u64
float, double	f32, f64
_Bool / bool	bool
enum	the matching signed integer
char * as a parameter or result	str
char * inside a struct or behind a pointer	ptr[char] (use str(p) to read it)
struct X by value	X, a struct with the same fields
struct X *, int *, …	ptr[X], ptr[i32], …
int arr[16] inside a struct	[16]i32: index it and use .len
void *, function pointers, incomplete structs	ptr

Example: structs and callbacks

import "raylib.h"
link "raylib"

fn main() {
    InitWindow(800, 450, "Finch")
    while !WindowShouldClose() {
        BeginDrawing()
        ClearBackground(Color(r: 30, g: 30, b: 40, a: 255))
        DrawCircleV(Vector2(400, 225), 50, Color(255, 136, 0, 255))
        EndDrawing()
    }
    CloseWindow()
}

To give C a function to call back (like qsort's comparator), use addr(myFunction). Its parameters must be C types (numbers, pointers, C structs).

What does not work yet

• C unions by value, and structs with bit fields by value (pointers to them work).
• Function-like macros and macros that are not a single number.
• long double, 128-bit integers.

---

18. Errors

Compile errors

game.fn:12:9: error: 'score' must be int, but this is str
   12 |     score = "high"
      |             ^

Format: file:line:column: error: explanation, then the line with a ^ under the spot.

Runtime errors

Instead of undefined behavior (as in C), Finch stops with a message and exit code 1:

Message	Cause
runtime error: division by zero	/ or % by a variable that was 0
runtime error: division overflows …	the smallest signed number divided by −1
runtime error: index 5 is out of range (the length is 3)	a[5] on a shorter array or str
runtime error: pop() on an empty array
runtime error: used .value on a null pointer	also .field and [i] through null
runtime error: can't turn "x" into int	int(...) / float(...) on text that isn't a number
runtime error: can't read the file "…"	read_file on a missing or unreadable file

---

19. Debugging

finch build game.fn -g -O0 -o game
gdb ./game
(gdb) break game.fn:12
(gdb) run
(gdb) bt                # where are we, through which functions
(gdb) info locals       # all variables
(gdb) print player.lives

-g adds debug info (lines, functions, variables, struct fields). -O0 keeps every variable visible; without it the optimizer may remove some.

---

20. Troubleshooting

Problem	Solution
can't find the C header 'x.h'	The library's development files are missing. Install them (Debian: libx-dev; Nix: add to shell.nix).
… 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.
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.
A program never ends	A loop condition never becomes false. Press Ctrl+C.

---

21. Project layout and tests

Finch/
├── src/            the compiler (C++)
├── runtime/        finch_rt.c: the small runtime (text, arrays, input, files)
├── boot/           the Finch compiler written in Finch (see the engineers' guide)
├── examples/       hello, tour, types, structs, todo, guess, c_import, window, raylib, llvm
├── tests/
│   ├── run/        programs + the exact output they must print (.out), input (.in)
│   ├── fail/       programs that must fail, with the expected error in line 1
│   ├── run.sh      the test runner (MEMCHECK=1 also checks memory with valgrind)
│   └── boot.sh     builds the self-hosted compiler with itself and compares
├── docs/           this documentation (en, pl)
├── shell.nix       the Nix development environment
└── CMakeLists.txt

tests/run.sh                 # → 58 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)

---

22. Current limits

Finch 2.0 is a complete small language, but not a finished one. Not there yet:

• Methods on structs, generics, maps/dictionaries, match. (Use arrays of structs and functions.)
• Error values: errors in int("x") or read_file stop the program. Check first (file_exists).
• Unicode-aware text: .len, s[i] and upper() work on bytes / ASCII.
• Threads.
• Platforms other than Linux x86-64.

See the roadmap in the README.