[Haskell-cafe] Type wildcards

Edsko de Vries devriese at cs.tcd.ie
Tue Dec 16 10:39:54 EST 2008


On Tue, Dec 16, 2008 at 05:26:00PM +0200, Eyal Lotem wrote:
> Martin Foster (aka. EvilTerran) suggested an interesting idea, and I decided
> it was too nice to ignore/be forgotten inside Martin's head... So I'd like
> to try and suggest it.
> Type wildcards that allow partially specifying types, e.g:
> f :: _ -> String
> f x = show x
> This will instruct the type-inferrer to "fill out" the wild-card part only
> (e.g: Show a => a).

What you are describing is (an instance of) the "some" operator. For
example, see


Section 5.1, "Partial annotations".

And yes, I agree, they are useful :)


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