[Haskell-beginners] Explanation of double astrix

[Bayley, Alistair]

> From: beginners-bounces at haskell.org 
> [mailto:beginners-bounces at haskell.org] On Behalf Of Paul Johnston
> 
> Hi
> Was playing around with ghci and lambda expressions and:
> 
> *Main> map (\x -> 2 * x) [1 ..3]
> [2,4,6]
> 
> Then thinking back to Fortran (yes I'm not young anymore!)
> 
> *Main> map (\x -> 2 ** x) [1 ..3]
> [2.0,4.0,8.0]
> 
> Curious as to what is going on.
> *Main> :t (\x -> 2 ** x)
> (\x -> 2 ** x) :: (Floating t) => t -> t
> *Main> :t (\x -> 2 * x)
> (\x -> 2 * x) :: (Num t) => t -> t
> 
> 
> Somehow the type has gone from Num to Floating
> I am using the excellent (IMHO) tutorial by Hal Daume and the 
> book by Graham 
> Hutton but can find no clues.
> 
> Cheers Paul


http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#
v%3A**

(**) is a function in the Floating class (Double and Float are instances
of this class).

(*) is a function in the Num class.

So, replacing (*) with (**) has changed the (Num t) constraint to a
(Floating t) constraint. Note that the type of your lambda is still t ->
t; it's just the class constraint which has changed (because (**) comes
from a different class than (*) ).

(The class hierarchy for Floating is: Num => Fractional => Floating, so
Floating has all of the methods of Fractional, and therefore Num).

Alistair

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