Disallow labels that match builtins or keywords #299
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Fixes dhall-lang/dhall-haskell#683 Before this change the grammar allowed variables to be named after builtins or keywords, such as: ```haskell let if = 1 in if ``` However, permitting this deteriorates error messages since an expression like this: ```haskell if a then b else ``` ... becomes a type error ("Unbound variable: if") instead of a parse error due to an incomplete `if` expression. The solution is to disallow these sorts of variable names. Given that we don't have a good way to do that in the grammar, we just specify it as a comment for now.
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Implements dhall-lang/dhall-lang#299 We add a negative lookahead in the simple-label rule, so that if a label starts with a keyword we backtrack. (Note: there's a space added, to avoid refusing partial matches, e.g. if a keyword is "in" we should still accept "index" as a label.
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Fixes dhall-lang/dhall-haskell#683
Before this change the grammar allowed variables to be named after
builtins or keywords, such as:
However, permitting this deteriorates error messages since an expression
like this:
... becomes a type error ("Unbound variable: if") instead of a parse
error due to an incomplete
ifexpression.The solution is to disallow these sorts of variable names. Given that
we don't have a good way to do that in the grammar, we just specify it
as a comment for now.