It is because of NoMomomorphismRestriction
let mTh4 = \x -> \y -> \z -> x * y * z :t mTh4 mTh4 :: Integer -> Integer -> Integer -> Integer :set -XNoMonomorphismRestriction let mTh4 = \x -> \y -> \z -> x * y * z :t mTh4 mTh4 :: Num a => a -> a -> a -> a
I'm not going into it too deeply, as it is somewhat involved and you can
read about it but I believe when a function "takes no arguments", it is
allowed to specialize polymorphic variables to defaults, and due to the Num
constraint it chooses Integer.
On Tue, Oct 17, 2017 at 1:28 PM, Wink Saville
I'm going through "Haskell Programming from first principles" and in section 7.3 Anonymous Functions there is an exercise on converting multiple parameters to anonymous functions, and it asks:
1. Which (two or more) of the following are equivalent?
mTh1 x y z = x * y * z mTh2 x y = \z -> x * y * z mTh3 x = \y -> \z -> x * y * z mTh4 = \x -> \y -> \z -> x * y * z
So I created a file, anon.hs (attached):
module Anon where
mTh1 x y z = x * y * z mTh2 x y = \z -> x * y * z mTh3 x = \y -> \z -> x * y * z mTh4 = \x -> \y -> \z -> x * y * z
I load that into ghci and check the function types:
$ ghci anon.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Anon ( anon.hs, interpreted ) Ok, 1 module loaded. *Anon> :t mTh1 mTh1 :: Num a => a -> a -> a -> a *Anon> :t mTh2 mTh2 :: Num a => a -> a -> a -> a *Anon> :t mTh3 mTh3 :: Num a => a -> a -> a -> a *Anon> :t mTh4 mTh4 :: Integer -> Integer -> Integer -> Integer
Why is mTh4 different from the rest?
On the flip side If I enter "mTh4 = \x -> \y -> \z -> x * y * z" directly in ghci command line then it has same type as the others:
$ ghci GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Prelude> mTh4 = \x -> \y -> \z -> x * y * z Prelude> :t mTh4 mTh4 :: Num a => a -> a -> a -> a
-- Wink
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