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Understand Compiler Diagnostics

Start with the first diagnostic in your source file. Later diagnostics can be caused by the same problem.

error[TYPE_MISMATCH]: Type mismatch
  --> program.casa:7:8
  |
7 | "text" consume_number
  |        ^^^^^^^^^^^^^^
  Expected: i64
  Got: str

Read it in this order:

  1. program.casa:7:8 identifies the file, line, and column.
  2. The carets identify the operation that detected the problem.
  3. Expected is what the operation needs at that stack position.
  4. Got is the value that was present.

The error code in brackets is useful for searching this reference, but the message and type details usually give the fix.

Fix stack and type errors

A stack effect lists consumed values from the top of the stack downward:

consume_number: i64 -> None

For a TYPE_MISMATCH, trace the values before the highlighted operation. Check which value is on top, its type, and the function's parameter order. Remember that a function's first parameter receives the topmost value.

STACK_UNDERFLOW means that an operation needs a value that is not present. Look for a missing literal, an earlier operation that consumed the value, or a branch that does not produce it.

STACK_MISMATCH means that continuing branches leave different stack states. Trace each if, match, or loop path from the same starting stack. Every path that continues must leave the same number and types of values.

SIGNATURE_MISMATCH means that a function body does not produce its declared stack effect. Compare the declared inputs and outputs with every return and with the end of the function body.

See Casa Guide for stack-effect notation and Functions and Lambdas for parameter order.

Expected, got, and inferred types

Some diagnostics use Inferred instead of Got. Inferred describes the stack effect or type calculated from the body. The same rule applies: compare it with Expected, then find the first position where they differ.

An integer or floating-point literal can also need more context. Add a type annotation when the surrounding operation cannot select a concrete width:

0 = offset: u64

Notes and related locations

A diagnostic can include one or more Note sections. A note points to a related declaration, branch, or value origin. The main location shows where the compiler detected the error. The note helps identify where the conflicting value or rule came from.

Cascade errors

The compiler can continue after some failures so that it can report more than one problem. A missing stack value is represented internally as <missing>. If a later diagnostic contains <missing>, fix the earlier underflow first. The later diagnostic will often disappear.

The compiler can also report independent errors from different functions or compilation phases. After each edit, compile again and start with the earliest remaining error in your code.

Error-code reference

Code Meaning
DUPLICATE_NAME A name or declaration is repeated
INVALID_SCOPE A construct appears in a scope where it is not allowed
INVALID_VARIABLE A binding or assignment is invalid
MISSING_TRAIT_METHOD A type does not satisfy a required trait
SIGNATURE_MISMATCH A function body does not match its declared stack effect
STACK_MISMATCH Control-flow paths leave incompatible stacks
STACK_UNDERFLOW An operation does not have enough input values
SYNTAX Invalid source form or unsupported construct
TRAIT_SIGNATURE_MISMATCH A trait declaration or implementation is incompatible
TYPE_MISMATCH A value has the wrong type
UNDEFINED_NAME A name cannot be resolved
UNEXPECTED_TOKEN A different token was required
UNMATCHED_BLOCK A block is missing its matching keyword

The CLI recognizes UNUSED_PARAMETER and LOSSY_TYPE_ANNOTATION warning codes. Warnings do not stop compilation.

Use the language server to see compiler diagnostics in an editor.