I was just playing around and noticed that the kind of the function
arrow in GHC is (?? -> ? -> *) when I (naively) expected it to be (* ->
* -> *).
After looking at
(http://hackage.haskell.org/packages/archive/ghc/6.10.2/doc/html/Type.html#5)
I see that the kind of (->) means that the parameter type cannot be an
unboxed tuple, whilst the result type can be anything. Why is this?
After reading this documentation I would expect the kind (? -> ? -> *).
I'm now wondering if the kind of (->) could cause problems if the
following style of declaration is requried:
> type FunArg a b = (a -> b, a)
*Main> :k FunArg
FunArg :: * -> * -> *
By using type variables, whose default kind is *, the function type is
fixed to use only boxed types. But if one wanted to allow unboxed type
parameters the kind would be wrong, and an explicit kind signature of #
or ? can't be given as they are not part of Haskell's source language.
I guess my question is why the (?? -> ? -> *) kind on (->) and what to
do if synonyms or data types over (->) are required such as the example
just stated. I'm just curious.
Thanks,
Dominic
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