FAQ and help for debugging `parse!` macro

[m4rw3r]

This is an issue collecting problems related to the parse! macro. Since the error messages are not very helpful from rustc when dealing with complex macros I will try my best to help here.

• Q: What does error: unexpected token:@__parse_internal ! { @ ACTION_NONTERM ( $ i , $ v : $ v_ty ) ; $ ( $ t ) * } } mean?

  A: An error has been encountered in a let-statement. Possible causes:

  • Syntax error on that line
  • Last line of a parse! macro-invocation. let-statements are not allowed as the final row of a parse! invocation since that prevents any return. Use an action without let (returns the value of the action itself) or use ret or err to return a success or error value respectively.

• Q: What does error: this function takes 1 parameter but 2 parameters were supplied [E0061] @ CONCAT ( $ f ( $ i , $ ( $ p ) , * ) , $ ( $ v ) * ) ; $ ( $ t ) * } } ; ( mean?

  A: An invocation of an action has too many or too few parameters listed in its argument list. parse! always prepends the input-context of the parser to every function invocation within parse!.

• Q: Are type-hints in function calls supported?

  A: No, but there is an issue

  Declare the type of the resulting variable instead:

  let var: MyDataType<Something> = my_action();
  

[dckc]

Ah! I wish I had found this sooner.

Yes, this is pretty important to have in the docs, since macros lead to such unhelpful compiler diagnostics.

[m4rw3r]

This part has now been moved to the parse! documentation: http://m4rw3r.github.io/chomp/chomp/macro.parse!.html#debugging