Metel Error Code Reference
All Metel errors carry a code. Codes are prefixed by phase:
| Prefix | Phase |
|---|---|
P | Parse — invalid source text |
T | Type — type-checker rejection |
R | Runtime — error during execution |
I | Internal — bug in the interpreter (please report) |
Parse errors (P)
P0001 — Syntax error
The source text does not match the Metel grammar.
[P0001] parse error in main.mtl at 12..18 (`let x = ;`): expected expression
Fix: correct the syntax at the indicated position.
P0002 — Invalid integer literal
An integer literal is out of range for i64 (−9,223,372,036,854,775,808 to 9,223,372,036,854,775,807).
[P0002] parse error in main.mtl at 4..24: integer literal '99999999999999999999' is out of range for i64
Fix: use a value that fits in i64, or split the computation.
P0003 — Invalid float literal
A float literal cannot be represented as an f64.
[P0003] parse error in main.mtl at 4..12: invalid float literal '1e9999'
Fix: use a value within the f64 range (~±1.8 × 10³⁰⁸).
Type errors (T)
T0001 — Type mismatch
Two types that must be equal are not.
[T0001] type error in main.mtl at 10..20: expected i64, got boolean
Fix: ensure the expression produces the expected type. Add an explicit cast if widening (e.g. x as f64).
T0002 — Annotation required
The type checker cannot infer a type without an explicit annotation.
[T0002] type error in main.mtl at 5..10: cannot infer type of `x`; add a type annotation
Fix: annotate the binding: let x: i64 = ....
T0003 — Undefined name
A name is used but not defined in the current scope.
[T0003] type error in main.mtl at 8..12: undefined name `foo`
Fix: define the variable or function before use, or correct the spelling.
T0004 — Arity mismatch
A function is called with the wrong number of arguments.
[T0004] type error in main.mtl at 5..20: expected 2 arguments, got 3
Fix: pass the exact number of arguments the function declares.
T0005 — Invalid operand types
A binary operator is applied to types it does not support.
[T0005] type error in main.mtl at 6..13: operator `+` cannot be applied to boolean and i64
Fix: use compatible types, or cast one operand.
T0006 — Assignment to immutable binding
A write operation targets a let binding. This covers three forms:
- Direct reassignment:
x = newValue - Field assignment through an immutable binding:
point.x = 1 - Taking a mutable pointer to an immutable binding:
&mut x
[T0006] type error in main.mtl at 3..12: `x` is immutable; use `mut x` to allow reassignment
Fix: change the binding declaration to let mut.
T0007 — Invalid cast
A as cast between incompatible types.
[T0007] type error in main.mtl at 5..15: cannot cast boolean to i64
Fix: only cast between numeric types (i64 as f64). Use an explicit conversion function for other types.
T0008 — Non-exhaustive match
A match expression does not cover all possible values of the scrutinee type.
[T0008] type error in main.mtl at 2..30: match on Colour is non-exhaustive; missing variant `Blue`
Fix: add the missing arms, or add a wildcard arm _ => ....
Runtime errors (R)
R0001 — No main function defined
Execution requires a main function but none was found.
[R0001] runtime error in main.mtl at 0..0: no main() function defined
Fix: add fn main() { ... } to your program.
R0002 — main is not a valid entry point
main exists but is generic or is not a function.
[R0002] runtime error in main.mtl at 0..0: main() is generic — not supported in v0.1
Fix: main must be a concrete, non-generic function with no parameters.
R0003 — Undefined variable at runtime
A variable name is not found in the current environment. This can occur when a variable is used before it is defined in a branch that the type-checker did not flag.
[R0003] runtime error in main.mtl at 10..15: undefined variable `x`
R0004 — Index out of bounds
An array index is negative or ≥ the array length.
[R0004] runtime error in main.mtl at 5..10: index 5 out of bounds (len 3)
Fix: check that the index is within 0..array.len() before access.
R0005 — Tuple index out of bounds
A tuple element is accessed by an index that does not exist.
[R0005] runtime error in main.mtl at 5..10: tuple index 3 out of bounds
Fix: tuple indices are fixed at compile time; verify the index against the tuple's declared length.
R0006 — Non-exhaustive match at runtime
A match expression reached its end without any arm matching. This indicates a pattern that the type checker approved as exhaustive but that is not, which is a known limitation.
[R0006] runtime error in main.mtl at 2..30: match: no arm matched scrutinee
R0007 — Arithmetic error
Integer division or remainder by zero.
[R0007] runtime error in main.mtl at 8..13: division by zero
Fix: guard with a zero check before dividing.
R0008 — Field not found
A struct or enum value does not have the accessed field.
[R0008] runtime error in main.mtl at 5..12: no field `colour` on value
Fix: check the field name against the type definition.
R0009 — Method not found
A method call cannot be resolved for the receiver type.
[R0009] runtime error in main.mtl at 5..20: no method `draw` on `Circle`
Fix: define the method in an impl block for the type.
R0010 — Call on non-callable value
A call expression (f(...)) is applied to a value that is not a function or closure.
[R0010] runtime error in main.mtl at 3..8: call: expected a closure or builtin
R0011 — Invalid for-in iterator
A for x in expr loop where expr does not evaluate to an Array or Range.
[R0011] runtime error in main.mtl at 1..20: for-in: expected Array or Range
Fix: ensure the iterable is an array literal, a range (a..b), or a variable of those types.
R0012 — Error propagation on non-Result value
The ? operator is applied to a value that is not a Result.
[R0012] runtime error in main.mtl at 5..10: ?: expected a Result value
Fix: only use ? on expressions whose type is Result[T, E].
Internal errors (I)
I0001 — Internal interpreter error
The interpreter reached an impossible state. This is a bug in the interpreter — the typechecker should have caught it before execution.
[I0001] internal error: binop: unsupported operand types (typechecker should have caught this)
What to do: please file a bug report at the Metel issue tracker with the source program that triggered this error.
I0002 — Not implemented
The program uses a language feature that is not yet supported in this version of the interpreter.
[I0002] internal error: generic functions are not supported in v0.1
What to do: check the changelog for the current supported feature set and the release plan for the planned implementation milestone.