On Thursday 30 September 2010 20:59:28, Russ Abbott wrote:
When I define zipWithPlus at the top level of GHCi, the type is as shown.
let zipWithPlus = zipWith (+)
zipWithPlus :: [Integer] -> [Integer] -> [Integer]
Why isn't it: (Num a) => a -> a-> a
It's the monomorphism restriction. If you bind a name without function arguments at the prompt (or at the top-level of a module if the name is exported) and without a type signature, it is given a monomorphic type. Type inference yields the polymorphic type, then it is monomorphised according to the defaulting rules. The default for Num constraints is Integer, hence you get zipWithPlus :: [Integer] -> [Integer] -> [Integer]
I was unable to find a way to get the type to be more general.
Three ways. 1. bind it with a type signature ghci> let zipWithPlus :: Num a => [a] -> [a] -> [a]; zipWithPlus = zipWith (+) ought to work (if it doesn't submit a bug report) 2. bind it with arguments ghci> let zipWithPlus xs ys = zipWith (+) xs ys and ghci> let zipWithPlus xs = zipWith (+) xs both ought to work (if not, bug report) 3. disable the monomorphism restriction $ ghci -XNoMonomorphismRestriction or ghci> :set -XNoMonomorphismRestriction disable the MR and give you the polymorphic type without type signatures or arguments It's probably a good idea to put :set -XNoMonomorphismRestriction into your ~/.ghci
I tried various declarations within the let using (Num a) => a -> a-> a but none of them were accepted.
Thanks. -- Russ Abbott