故障映射功能与操作员（/ x）

Question

So i have the result of a function that given a list of ints subtractes an int to all of the numbers in the list and then i want to divide the new list by x in this case 12. If i do the first paragraph of coding it gives me an error but if i do the second one it is possible. How do i do this and why does it give me an error?

let xs = [23,32,1,3]
map (/12) xs

map(/12) [23,32,1,3]

potenciasPor12 xs = map (/12) xs

This is the error i'm getting

<interactive>:176:1:
No instance for (Fractional Int)
  arising from a use of ‘potenciasPor12’
In the expression: potenciasPor12 xs
In an equation for ‘it’: it = potenciasPor12 xs

Answer 1

If the monomorphism restriction is set (it is off by default in newer GCHi, but on in compiled code), then xs will default to [Int] rather than the more general type Num a => [a] which would work the the (/) operator.

(In GHCi 8.4.1, at least, it appears to default to Integer instead of Int .)

% ghci
GHCi, version 8.4.1: http://www.haskell.org/ghc/  :? for help
Prelude> let xs = [1,2]
Prelude> :t xs
xs :: Num a => [a]
Prelude> :set -XMonomorphismRestriction
Prelude> let ys = [1,2]
Prelude> :t ys
ys :: [Integer]

Always provide explicit type signatures to be sure:

% ghci -XMonomorphismRestriction
GHCi, version 8.4.1: http://www.haskell.org/ghc/  :? for help
Prelude> let xs = [23,32,1,3] :: Num a => [a]
Prelude> :t xs
xs :: Num a => [a]
Prelude> map (/12) xs
[1.9166666666666667,2.6666666666666665,8.333333333333333e-2,0.25]