(WIP, RFC) Distinguish variable-not-found-error from others#35
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Problem ==== - Handling `Maybe` values in the `Parser` monad is troublesome. - Even `envMaybe` mixes up variable-not-found-error with others, which makes the difference of `Nothing` and `Left String` obscure. Solution ==== Change the error type of `Parser a` from just a `String` into a dedicated error ADT `ParseError`. NOTE ==== - Fix dmjio#30 as a bonus!
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Problem
Maybevalues in theParsermonad is troublesome.envMaybemixes up variable-not-found-error with others,which makes the difference of
NothingandLeft Stringobscure.
Solution
Change the error type of
Parser afrom just aStringintoa dedicated error ADT
ParseError.NOTE