[Haskell-cafe] Arithmetic expressions with GADTs: parsing
j.romildo at gmail.com
j.romildo at gmail.com
Wed May 2 13:03:45 CEST 2012
Hello.
In order to learn GADTs, I have written the attached program, which
defines a type for arithmetic expressions using GADTs, a parser for
them, and an evaluation function.
But my parser does not typecheck. ghc-7.4.1 gives me the error message:
Expr.hs:25:28:
Couldn't match expected type `Double' with actual type `Bool'
Expected type: Bool -> Expr Double
Actual type: Bool -> Expr Bool
In the first argument of `(<$>)', namely `B'
In the first argument of `(<|>)', namely `B <$> pBool'
Any clues on how to fix that?
Romildo
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