Good to know extension. And without it? On Tue, Aug 13, 2013 at 4:59 PM, Brandon Allbery wrote:

On Tue, Aug 13, 2013 at 10:46 AM, Lukas Lehner wrote:

...

That means ghc cannot infer the type. Is there a way how to # print flatten (List []) ? Or even more general, print [] without enforcing the type?

If you turn on the ExtendedDefaultRules extension ( `{-# LANGUAGE ExtendedDefaultRules #-}` pragma or `-X ExtendedDefaultRules` ghc option), ghc will infer () for the type just as ghci does. Note that this reduces type safety a bit, since ghc will now accept programs that have what otherwise would be type errors.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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That I understand. My question is that I have to enforce the type that specific way (don't want to use ExtendedDefaultRules) or is there some more generic way? On Wed, Aug 14, 2013 at 12:08 PM, David Virebayre < dav.vire+haskell@gmail.com> wrote:

2013/8/13 Lukas Lehner :

...

Good to know extension. And without it?

Someone correct me if I'm wrong, here's how I understand it:

To execute print [], ghc has to find which instance of Show to call. How could it find an instance without knowing the type ? [] could be a list of anything, and while in theory we know that in all cases it should print "[]", in practice ghc can't call that code if it doesn't know the type of the list.

David.

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There is a way. It's print "[]" if the compiler cannot infer the type of an empty list, it's because you wrote a literal empty list []. Sorry for giving such a practical (non-theoretical) answer ;) Michael