[Haskell-cafe] Re: type class question
ben.franksen at online.de
Wed Dec 5 19:38:24 EST 2007
Brent Yorgey wrote:
> Well, first of all, the definition of compCommand should use calls to
> compBlock, not recursive calls to compCommand. But that's not the main
> source of your problems.
> What exactly are you trying to accomplish? And why do you need a type
Whatever the code is supposed to accomplish, there is something strange
going on with the type checking. I have managed to reduce the code (while
keeping the type error message) thus
data Command = Skip
class Java block command where
block_ :: [command] -> block
compBlock :: [Command] -> block
--compBlock = block_ . map compCommand
compCommand :: Command -> command
This compiles ok. But when I ask ghci for the type of the body of the
default definition of compBlock I get
*Main> :t block_ . map compCommand
block_ . map compCommand :: forall block block1 command.
(Java block command, Java block1 command) =>
[Command] -> block
and if I remove the comment from the default definition of compBlock I get
Could not deduce (Java block command1)
from the context (Java block command)
arising from use of `block_' at Bla.hs:7:14-19
add (Java block command1)
to the class or instance method `compBlock'
In the first argument of `(.)', namely `block_'
In the expression: block_ . (map compCommand)
In the definition of `compBlock':
compBlock = block_ . (map compCommand)
It would be nice if someone could explain (in language that can be
understood by non-type-system-experts) why ghc(i) deduces these
strange 'duplicated' contexts.
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