Haskell forall error

Question

Eclipse show error in editor at explicit forall, though I can run the script without errors. How can I fix this? (This also happens when I run it in cmd, think I need a flag here!?)

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Also warnings about defaulting to integer type are annoying is there a way to stop them?

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Have you enabled `{-# LANGUAGE ExplicitForall #-}` within your file, or where? (Or `ScopedTypeVariables`) — leftaroundabout, Feb 18 '14 at 11:55
I should define this in top of the file? Should I define it in each file?! — Karim Tarabishy, Feb 18 '14 at 12:00
What do you need the `forall` for? Unless you don't have a specific need for it you might want to remove it. — Danny Navarro, Feb 18 '14 at 12:14
@Doggynub: you certainly need to enable it _somewhere_, the usual options being either in your `project.cabal` file, or on top of each source file. — leftaroundabout, Feb 18 '14 at 12:16
@DannyNavarro: The `forall` is probably suggested by an hlint quick fix - my guess is for a missing type signature. — yatima2975, Feb 18 '14 at 15:07
@Doggynub the `forall` is implied here (essentially) and doesn't really do anything in this case. — David Young, Feb 18 '14 at 19:00

score 2 · Accepted Answer · answered Feb 18 '14 at 12:51

2

You need to explicitly declare the language extension in each file like this

{-# LANGUAGE FOO #-}

where FOO is either, ExplicitForall which just let's you write forall, ScopedTypeVariables which means that you can write

 foo :: forall a. a -> [a]
 foo a = [a] :: [a]

and have the explicit signature work as expected. Or RankNTypes which let's you write types of a higher rank like

 foo :: (forall a. a -> a) -> Int -> Int

In this case it looks like you just want ExplicitForall.

answered Feb 18 '14 at 12:51

daniel gratzer

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about the defaulting to integer thing It seems you need to remove the -Wall option or specify type of all numbers `1::Int` --- just if any one want to know – Karim Tarabishy Feb 18 '14 at 14:07
@Doggynub I'm not able to replicate that warning so I can't help you there – daniel gratzer Feb 18 '14 at 14:30

Haskell forall error

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