Haskell's Test.QuickCheck module exports pattern Fn, which I have been using.

When I import it with:

import Test.QuickCheck

it works fine. However, when I import it with:

import Test.QuickCheck (Fn)

I get an error: Module ‘Test.QuickCheck’ does not export ‘Fn’. I have also tried import Test.QuickCheck (Fun(Fn)), but get a similar error doing that.

As mentioned, the blanket import works, but here my preference is to use explicit imports so that I can easily see where each imported term came from. Is it possible to import such 'patterns' explicitly?

CodePudding user response：

You can't do import Test.QuickCheck (Fun(Fn)) because, although Fn does construct a Fun value, it is not actually one of the associated constructors of this type. Rather, it is a standalone pattern synonym. As such, you need the corresponding extension to explicitly import it:

{-# LANGUAGE PatternSynonyms #-}
import Test.QuickCheck (pattern Fn)