[Haskell-beginners] Problem with GADTs and Parsec

[Brandon Allbery]

On Sat, Feb 8, 2014 at 2:10 PM, Lorenzo Tabacchini <lortabac at gmx.com> wrote:

> If I do:
>
> exprParser :: Parsec String u (Expr a)
> exprParser = parens exprParser
>     <|> (reserved "true" >> return (Bool True))
>     <|> (reserved "false" >> return (Bool False))
>     <|> (stringLiteral >>= return . Text)
>
> The compiler says: "Couldn't match type `Bool' with `[Char]'"
> Where am I wrong?
>

The type of exprParser doesn't do what you intend. Specifically, the `a` in
`(Expr a)` is chosen by the *caller* and you have to deal with that choice
somehow --- it does not mean you can pick a different `a` in different
parts of your code.

There are several ways you might redesign your types, but I think I'll have
to let someone else address those.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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