CODEWITHSUNDEEP
haskellpermutationghci
user2832640
user2832640

Reputation:

Haskell Data.List permutations of functions

How to fix the permutation error on a list of functions? 

> :m + Data.List

> permutations [(+1),(-2),(*3)]

No instance for (Num a0) arising from a use of `+'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
  instance Num Double -- Defined in `GHC.Float'
  instance Num Float -- Defined in `GHC.Float'
  instance Integral a => Num (GHC.Real.Ratio a)
    -- Defined in `GHC.Real'
  ...plus three others
In the expression: (+ 1)
In the first argument of `permutations', namely
  `[(+ 1), (- 2), (* 3)]'
In the expression: permutations [(+ 1), (- 2), (* 3)]

Upvotes: 1

Views: 376

Answers (1)

daniel gratzer
daniel gratzer

Reputation: 53891

In Haskell the type of 1 is

1 :: Num a => a

So Haskell can't decide what a to choose in your case. You can fix this with a simple type signature

perms :: Num a => [[a -> a]]
perms = permutations [(+1),(subtract 2),(*3)]

Upvotes: 3

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