On Thu, Nov 12, 2009 at 9:37 AM, Dag Hovland
Hi!
I have a problem with understanding some types given by ghc and hugs. The file loaded is:
f1 = \x -> x * 2 f2 x = x * 2
After they are loaded I get the following from ghci
*Main> :t f1 f1 :: Integer -> Integer *Main> :t f2 f2 :: (Num a) => a -> a *Main> :t \x -> x * 2 \x -> x * 2 :: (Num a) => a -> a
This is called the monomorphism restriction, it's a rule that state a binding _without_parameters_ shall be inferred of a monomorphic type unless an explicit signature is given. There are several reasons for it, some of efficiency (polymorphism has a cost) and some of a more technical nature, refer to the Haskell Report for a more detailed explanation. Some Haskellers think this restriction is no longer required, that GHC is now often intelligent enough to alleviate the cost of polymorphism, that the technical reasons are not really all that pertinent and that the default should be to infer the more general type in all case rather than confuse beginners and oblige experts to put explicit signatures. It is already possible to deactivate the restriction by using the -XNoMonomorphismRestriction argument (or putting the equivalent language pragma in the code itself or in the cabal description file) and making this the default is discussed for Haskell' (the future standard for Haskell). In the meantime, it is a good idea to put ":set -XNoMonomorphismRestriction" in your .ghci file since most usage of GHCi would only hit the disadvantages of this rule and reap no benefits from it. -- Jedaï