CODEWITHSUNDEEP
haskelltypescompiler-errors
CYC
CYC

Reputation: 629

Why am I getting a "No instance for (Fractional a0) ..." error?

The following code doesn't work when I type test 10:

test m = if m `mod` 2==0
             then m/2
             else m

It says the following:

No instance for (Fractional a0) arising from a use of ‘it’
The type variable ‘a0’ is ambiguous
Note: there are several potential instances:
  instance Integral a => Fractional (GHC.Real.Ratio a)
    -- Defined in ‘GHC.Real’
  instance Fractional Double -- Defined in ‘GHC.Float’
  instance Fractional Float -- Defined in ‘GHC.Float’
In the first argument of ‘print’, namely ‘it’
In a stmt of an interactive GHCi command: print it

It might be some integer or double type problems in test n for integer n, but I don't know what's wrong.

Upvotes: 1

Views: 321

Answers (1)

melpomene
melpomene

Reputation: 85827

The problem is that mod only works for integer types but / only works for fractional types, so there's no way to actually use your test function.

You can do this instead:

test m = if m `mod` 2 == 0
             then m `div` 2
             else m

(div is integer division.)

Or this:

test m | even m    = m `div` 2
       | otherwise = m

Or this:

test m
    | (d, 0) <- m `divMod` 2 = d
    | otherwise              = m

Upvotes: 7

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