Type inference

Not a single type is written down, and every one of them is still checked.

W# is Zig-flavoured with one deliberate departure: type annotations are optional everywhere. They are checked when written and inferred when not. This program writes none at all.

// Type inference.
//
// Not a single type is written down here. Hindley-Milner works all of them out,
// and `wsharp check examples/inference.ws --emit=types` will show you what it
// decided:
//
//     add:    fn(i64, i64) i64
//     scale:  fn(f64) f64
//     id:     fn(T) T
//     first:  fn(T, U) T
//     main:   fn() i64

fn add(a, b) { return a + b; }

// `x * 2.0` forces f64, and that flows out through the return type.
fn scale(x) { return x * 2.0; }

// Nothing constrains `x`, so `id` is generic. Each use below compiles to its
// own specialised copy of the function.
fn id(x) { return x; }

fn first(a, b) { return a; }

fn main() i64 {
    print(add(20, 22));
    print(scale(1.5));

    print(id(7));
    print(id(true));
    print(id("generic"));

    print(first(1, "ignored"));
    return 0;
}

Ask the compiler what it decided:

wsharp check examples/inference.ws --emit=types

add: fn(i64, i64) i64
scale: fn(f64) f64
id: fn(T) T
first: fn(T, U) T
main: fn() i64

This is Hindley-Milner inference over the whole program, so fn add(a, b) has a signature rather than a hope. --emit=types is the fastest way to find out what the compiler thinks, and it is worth reaching for whenever a type error surprises you.

Where each of those came from

add uses +, which is defined on the numeric types, and nothing narrows it further, so the integer literals at the call site settle it: + defaults to i64.

scale multiplies by 2.0. A float literal is an f64 and nothing else, so x must be one too, and that flows out through the return type.

id puts no constraint on its argument at all, so it generalises to fn(T) T. Because values are unboxed, this cannot be compiled once. Inference records the type arguments at each call site and a later pass emits one copy per instantiation, so id(7), id(true) and id("generic") are three functions in the emitted program, and any instantiation nothing calls is never emitted at all.

first ignores its second argument, which is why the second type variable never appears in the result.

Three things that follow

An integer literal takes the type it is used at, and defaults to i64 when nothing says otherwise. So const b: u8 = 255; needs no conversion, and 0xff is a u8 in a u8 context and an i64 in an i64 one.

A float literal is never an integer, and an integer literal may be a float. const x: f64 = 1; is legal, and so is a 1 in a body annotated Number that gets compiled at f64 as well as at i64. Only for a value the f64 is: above 2^53 the integers are no longer all representable, and the check is the round trip rather than a range, because rounding a written constant silently is not a thing to do.

Conversions are written, never inferred. u32(x) converts. There is no implicit widening between numeric types, so a mismatch is an error you fix by saying which type you meant.

Annotations still work, and are still checked

Writing a type down does not turn inference off; it adds a constraint that has to hold.

fn add(a: i64, b: i64) i64 { return a + b; }   // the same function
fn add(a: i64, b) { return a + b; }            // b is i64 too, by unification
fn add(a: f64, b) i64 { return a + b; }        // error: f64 is not i64

Annotate for the reader, or to pin a type that would otherwise default. Do not annotate because the compiler needs it, because usually it does not.

When it cannot decide

Two cases come up in practice.

A field read needs a known type. Structs are nominal and there is no row polymorphism, so fn getx(p) { return p.x; } cannot be inferred: any struct with an x would satisfy it, and W# has no way to say that. Annotate the parameter.

A generic call with nothing to pin it. var b = array.new(32); says how many elements but not of what, and the error says cannot tell what type main is being used at. Give it something to work from, such as a use of b that fixes the element type, or an annotation.

How it is organised, briefly

Top-level functions are grouped into strongly connected components of the call graph, inferred with their types held monomorphic, and generalised only once the whole group is done. That is what makes fib calling itself check, and mutually recursive functions too. An overload set lands in one group, so its members generalise together. There is more of this in the internals.

Last changed 11 September 2026. Improve this page