Thanks for the quick reply - I'm not sure I really understand exactly what your rule means, but it's led me over to http://www.haskell.org/haskellwiki/Monomorphism_restriction where it seems possible that I might eventually figure it out :) Any thoughts on the other question about where I can go to understand Haskell's precedence/associativity rules better (and avoid so many parens)? Cheers, Patrick -----Original Message----- From: Debian User [mailto:mdanish@andrew.cmu.edu] Sent: Wednesday, April 02, 2008 4:40 PM To: Patrick Surry Cc: glasgow-haskell-users@haskell.org Subject: Re: Confused about typing in ghci On Wed, Apr 02, 2008 at 04:26:04PM -0400, Patrick Surry wrote:
-- Why don't these (particularly g and g') all have the same type? Prelude> :t (\x -> x+1) (\x -> x+1) :: (Num a) => a -> a Prelude> let g = (\x -> x+1) Prelude> :t g g :: Integer -> Integer Prelude> let g' x = x + 1 Prelude> :t g' g' :: (Num a) => a -> a
It's the monomorphism restriction in action, along with defaulting. Basically, the rule is: Any binding of the form "g = ..." where there are no parameters *syntactically* must not be polymorphic with type-class restrictions. If you disable it with -fno-monomorphism-restriction then the differences go away. In addition, Haskell has some special-case "defaulting" rules for turning Num classed type-variables into concrete types, which explains the appearance of "Integer" specifically. The reason for existence of this restriction is that such a "g" may be recomputed for each use, much like a function call. This could lead to surprising behavior, though perfectly safe. In Haskell, it's not such a big deal, because of pure code, so some people prefer to turn the restriction off. Your options are to turn it off, always write syntactic parameters like "g x = ...", or provide an explicit polymorphic type signature like "g :: Num a => a -> a". -- -- Matthew Danish -- user: mrd domain: cmu.edu -- OpenPGP public key: C24B6010 on keyring.debian.org ________________________________________________________________________ DISCLAIMER: This e-mail is intended only for the addressee named above. As this e-mail may contain confidential or privileged information, if you are not the named addressee, you are not authorised to retain, read, copy or disseminate this message or any part of it. If you received this email in error, please notify the sender and delete the message from your computer.