spec > core > declarations

declarations and bindings

Status: α1

declarations and bindings

this covers how you declare variables and what happens when you assign them.

immutability by default

the default binding is const:

const pageSize: usize = layout.page_size();

const bindings can't be reassigned. this is the common case and should be your default.

mutable bindings

use var when you need mutation:

var counter: u32 = 0;
counter = counter + 1;

type inference

ferrule infers types when unambiguous:

const x = 42;        // i32 (default integer)
const y = 3.14;      // f64 (default float)
const s = "hello";   // String
const b = true;      // Bool

annotation required when ambiguous or non-default:

const port: u16 = 8080;      // could be many int types
const ratio: f32 = 3.14;     // need f32 not f64
const items: Vec<User> = vec.new();  // empty collection needs type

function results always need annotation:

const result = compute();       // error: can't infer, annotate
const result: Data = compute(); // ok

literal type preservation

const preserves literal types, var widens:

const x = 42;     // type is literal 42
var y = 42;       // type is i32 (widened)

this matters for const generics and refinement types.

no implicit coercion

ferrule never converts types implicitly:

const a: u16 = 100;
const b: u32 = a;       // error: u16 is not u32

const b: u32 = u32(a);  // ok: explicit conversion

this applies to everything:

• no int to float
• no narrowing (i32 to i16)
• no widening (i16 to i32)
• no bool coercion
• no null coercion

move semantics

when you assign a move type, ownership transfers:

const s: String = "hello";
const t = s;      // s is moved to t
// s is now invalid

this is not a copy. the data isn't duplicated. s becomes unusable after the assignment.

for copy types, assignment duplicates:

const a: i32 = 42;
const b = a;      // a is copied to b
// both a and b are valid

see types.md for which types are copy vs move.

use after move

using a moved value is a compile error:

const data: String = "hello";
const other = data;  // move

io.println(data);  // error: data was moved

the compiler tracks which variables have been moved and errors if you try to use them.

conditional moves

if a value might be moved in one branch, it's invalid after the conditional:

const data: String = "hello";

if condition {
    consume(data);  // moves data
}

use_data(data);  // error: data might have been moved

the safe pattern is to move in all branches:

const data: String = "hello";

if condition {
    consume(data);
} else {
    other_consume(data);
}
// data is invalid on all paths, which is fine

loop moves

you can't move the same variable in a loop:

const data: String = "hello";

for i in 0..3 {
    process(data);  // error: can't move in loop
}

use clone if you need to pass owned data in each iteration:

for i in 0..3 {
    process(data.clone());  // explicit copy each time
}

by-reference parameters

use inout for by-reference mutation:

function bump(inout x: u32) -> Unit { 
  x = x + 1; 
}

var counter: u32 = 0;
bump(inout counter);
// counter is now 1

rules:

• inout is only valid on function parameters
• callers must pass mutable bindings
• the mutation is visible to the caller

destructuring

you can destructure records and arrays:

const User { name, age } = user;  // moves both fields out
// user is now fully invalid

const [first, second, ...rest] = items;  // array destructuring

partial moves (moving just one field) are not allowed. if you need one field, destructure the whole thing.

summary