On 7/31/07, Chris Eidhof
Hey Haskell-Cafe,
I was trying out the code in Dons's article [1], and I noticed a weird thing when doing it in GHCi. When binding the function composition to a variable, the type suddenly changes. I'm not completely sure why this happens. Is this because GHCi is in a monad and wants to find an instance for the type variable? Here's my GHCi session: [ ... ]
Apfelmus already explained why it happens. I'd like to add one thing. If you are experimenting with GHCi, you can turn the restriction off with the option -fno-monomorphism-restriction. In GHCi itself this is given as follows: Prelude> :set -fno-monomorphism-restriction After giving this and entering the same things as in your original message, the type of encode came out to be as follows: Prelude Control.Arrow Data.List> let encode = map (length &&& head) . group Prelude Control.Arrow Data.List> :t encode encode :: (Eq a) => [a] -> [(Int, a)] This option also saves some typing (of the variety you do on the keyboard!) when you just want to use GHCi as a calculator: Prelude Control.Arrow Data.List> let x = 5 Prelude Control.Arrow Data.List> let y = 6.3 Prelude Control.Arrow Data.List> x*y 31.5 Instead of Integer, the type of x is now x :: (Num t) => t without the restriction, and I don't need to add fromInteger to the multiplication. I don't recommend you to use this option all the time, of course. It's just a convenience. Pekka