Question

I need to use the map function to get for example pence to pounds conversion.
Sorry for the stupid question.. but I am a beginner.

del :: Int -> Float
del x =  ( fromIntegral x ) / 100

pounds :: [Int] -> [Float]
pounds = map del 

and I get this error..

*Main> pounds 45
<interactive>:90:8:
    No instance for (Num [Int])
      arising from the literal `45'
    Possible fix: add an instance declaration for (Num [Int])
    In the first argument of `pounds', namely `45'
    In the expression: pounds 45
    In an equation for it': it = pounds 45

Answer 1

It seems you typed

ghci> pounds 45

at the prompt. But pounds expects a list (of Int) as its argument. You should either use

ghci> del 45

there, or

ghci> pounds [45]

Since integer literals have an implicit fromInteger, GHC tries to find the conversion fromInteger :: Integer -> [Int], which would require an instance Num [Int], but it can’t find one, that’s the error it reports.