Debugging: CLI Tips and Common Errors

Rex is compiled and evaluated in stages:

1. Lexing
2. Parsing
3. Type inference / checking
4. Evaluation

Most debugging is about figuring out which stage is failing and adding just enough information to make the problem obvious.

Useful CLI flags

Run a file:

cargo run -p rex -- run path/to/file.rex

Run an inline snippet:

cargo run -p rex -- run -c 'let x = 1 in x + 2'

Show the parsed AST and exit:

cargo run -p rex -- run --emit-ast -c '1 + 2'

Show the inferred type and exit:

cargo run -p rex -- run --emit-type -c 'map ((*) 2) [1, 2, 3]'

“Parse error”: start small

If you hit a parse error:

1. Reduce the program to the smallest failing snippet.
2. Add parentheses to disambiguate application vs infix operators.
3. Prefer layout-style let/match while debugging.

“Missing typeclass impl”

This usually means you called a type-class method at a type that has no instance.

Typical fixes:

• use a different type (List vs Array, Option vs Result, …),
• add an instance (Section 2),
• add a type annotation so the intended instance is selected.

“Ambiguous overload”

This happens when an overloaded value doesn’t have enough information to pick an instance.

Typical fixes:

• add a let-annotation: let z: i32 = zero in z
• add is ascription: (zero) is i32 (if you prefer expression ascription style)
• use the value in a context that forces a type (e.g. zero + 1).

The exact defaulting rules are described in Specification.