Type wildcards, was: Re: [Haskell] GHC Error question

Johannes Waldmann waldmann at imn.htwk-leipzig.de
Thu Dec 7 07:20:11 EST 2006

Lennart Augustsson wrote:

> Btw, this reminds me again that I'd like to be able to use _ in type
> signatures.
> With the meaning of _ being, "there's a type here, but I can't be
> bothered to write it out in full."

you're not alone ...

what is the meaning of two _ in one expression?
do they necessarily denote the same type? (probably not.)

what about type classes? for example,
given  sort :: Ord a => [a] -> [a],
would it be ok to write   sort :: [ _ ] -> [ _ ]
(that is, omitting the context)
or is it more like  sort :: _ => [ _ ] -> [ _ ]

BTW, this "_ for context" would be useful on its own!
I have several cases where the conxtext for the type decl
is longer than the implementation of the function. Well, nearly.

With previous ghc versions, I could get this effect
by omitting the type decl but writing
f ( x :: type1) ( y :: type2 ) = ...

I sometimes wish haddock would understand that. (*)
Because - if you write a haddoc comment for a type declaration,
then you don't have names for the function's arguments.
This leads to either awkward prose (random example, from Data.Maybe:)

> The maybe function takes a default value, a function, and a Maybe
> value. If the Maybe value is Nothing ...

(this only works here because all the argument types are different in
this case) or you have to re-invent names, random example:

> approxRational :: RealFrac a => a -> a -> Rational
> approxRational, applied to two real fractional numbers x and epsilon,

which looks like duplication of work.

(*) of course the full form would include the return type

fun (x :: type1 ) (y :: type2) :: type3 = ...

I expect strong opposition from those who write
functions with pattern matching (then there is a group of declarations
and which one should be type-annotated?) but that's bad style anyway
since constructors should not be exported :-)

Best regards,
-- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 --
---- http://www.imn.htwk-leipzig.de/~waldmann/ -------

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