CODEWITHSUNDEEP
parsingsly
Sean Duggan
Sean Duggan

Reputation: 1135

How can I indicate an error during a parse operation?

Within the scripting language I am implementing, valid IDs can consist of a sequence of numbers, which means I have an ambiguous situation where "345" could be an integer, or could be an ID, and that's not known until runtime. Up until now, I've been handling every case as an ID and planning to handle the check for whether a variable has been declared under that name at runtime, but when I was improving my implementation of a particular bit of code, I found that there was a situation where an integer is valid, but any other sort of ID would not be. It seems like it would make sense to handle this particular case as a parsing error so that, e.g., the following bit of code that activates all picks with a spell level tag greater than 5 would be considered valid:

foreach pick in hero where spell.level? > 5
  pick.activate[]
nexteach

but the following which instead compares against an ID that can't be mistaken for an integer constant would be flagged as an error during parsing:

foreach pick in hero where spell.level? > threshold
  pick.activate[]
nexteach

I've considered separate tokens, ID and ID_OR_INTEGER, but that means having to handle that ambiguity everywhere I'm currently using an ID, which is a lot of places, including variable declarations, expressions, looping structures, and procedure calls.

Is there a better way to indicate a parsing error than to just print to the error log, and maybe set a flag?

Upvotes: 0

Views: 338

Answers (1)

James K. Lowden
James K. Lowden

Reputation: 7837

I would think about it differently. If an ID is "just a number" and plain numbers are also needed, I would say any string of digits is a number, and a number might designate an ID in some circumstances.

For bare integer literals (like 345), I would have the tokenizer return maybe a NUMBER token, indicating it found an integer. In the parser, wherever you currently accept ID, change it to NUMBER, and call a lookup function to verify the "NUMBER" is a valid ID.

I might have misunderstood your question. You start by talking about "345", but your second example has no integer strings.

Upvotes: 1

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