[Haskell-cafe] using type variables in type declarations inside function

Brandon Allbery allbery.b at gmail.com
Thu Apr 12 19:51:40 UTC 2018

On Thu, Apr 12, 2018 at 3:47 PM, Dennis Raddle <dennis.raddle at gmail.com>

> myFunc :: a -> b -> c
> myFunc x y z = ...
>   where
>     helper :: a -> [b]
>     helper xx = ...
> Notice that I'm attempting to declare 'helper' using my type variables.
> I've noticed that this results in an error.
> Is this actually possible, and how?

You need the ScopedTypeVariables extension, *and* to "declare" the type
variables whose scope is to be extended with an explicit "forall" in the

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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