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Error Messages

constellation-lang provides precise error messages with line and column information.

Debugging Strategy

When you encounter an error, check the line and column numbers first. The error location points to where the problem was detected, which is often (but not always) where the fix should be applied. For type errors, trace back to where the variable was defined.

Undefined Variable​

out undefined_var
Error at 1:5: Undefined variable: undefined_var

Undefined Type​

in x: NonExistent
Error at 1:6: Undefined type: NonExistent

Undefined Function​

result = unknown_func(x)
Error at 1:10: Undefined function: unknown_func

Type Mismatch​

# If function expects Int but receives String
result = expects_int(stringValue)
Error at 1:22: Type mismatch: expected Int, got String

Invalid Projection​

in data: { id: Int, name: String }
result = data[id, nonexistent]
Error at 2:10: Invalid projection: field 'nonexistent' not found. Available: id, name

Incompatible Merge​

in a: Int
in b: String
result = a + b
Error at 3:10: Cannot merge types: Int + String
Common Merge Confusion

The + operator in constellation-lang is for merging record types, not numeric addition. For arithmetic, use function calls like add(a, b) from stdlib.math. Merge only works between compatible record types.

Parse Errors​

in x: Int
out @invalid
Error at 2:5: Parse error: expected identifier
Check Syntax Carefully

Parse errors often indicate a syntax issue on the previous line, such as a missing comma or closing bracket. If the indicated line looks correct, check the line above it.

  • Types — Type system reference and common type errors
  • Expressions — Valid expression syntax
  • Type Algebra — Record merge and projection rules