Error passing parameters through haskell function

Question

I'm receiving an error and don't know why.

mapo :: ((Float, Float) -> Float) -> [(Float, Float)] -> [Float]
mapo f [] = []
mapo f (x:y) = f x : mapo f y

Compiles ok.

*Main> mapo + [(1,2)]

Couldn't match type `(Float, Float) -> (Float, Float)' with `Float'
Expected type: (Float, Float) -> Float
  Actual type: (Float, Float) -> (Float, Float) -> (Float, Float)
In the first argument of `mapo', namely `(+)'
In the expression: mapo (+) [(1, 2)]
In an equation for `it': it = mapo (+) [(1, 2)]

The purpose of this function is to receive an operator (let's say + or -), a list of pairs of numbers (Float) and return the result of using that operator with each of the pairs of parameters of the list.

Please change the title of your question. "What am I doing wrong" doesn't tell anyone anything about the contents of your question. — Cubic
– Cubic, Commented Jul 13, 2015 at 21:08

Cubic · Accepted Answer · 2015-07-13 21:10:28Z

3

Your problem is merely a syntactic error; When you want to partially apply an operator you need to wrap it in parentheses; So instead of

mapo + [(1,2)]

write

mapo (+) [(1,2)]

Otherwise it parses as adding mapo to [(1,2)] (i.e. (+) mapo [(1,2)] which is clearly not what you want.

Next thing is getting the type right:

(+) :: Float -> Float -> Float (in your case, its real type is more general) - but your function wants (Float,Float) -> Float. These two types aren't equal, but they express pretty much the same thing so you can easily map between them with the curry :: ((a,b) -> c) -> a -> b -> c and uncurry :: (a -> b -> c) -> (a,b) -> c functions. In this case you need to pass uncurry (+) to your mapo function.

answered Jul 13, 2015 at 21:10

Cubic

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3 Comments

Cubic Over a year ago

It won't. That's what the second part of the answer is for.

Hernan Over a year ago

Thanks @Cubic! Another question. Lets say that i dont wont to use uncurry and i can change the function...if i change it to mapo::(Float->Float->Float)->[(Float,Float)]->[Float] Im receiving an error at compile time. How would you do it?

Cubic Over a year ago

Just do the uncurry in mapo.

ErikR · Accepted Answer · 2015-07-13 21:12:15Z

1

(+) does not have type (Float, Float) -> Float, but uncurry (+) does:

mapo (uncurry (+)) [(1,2)]

-> [3]

answered Jul 13, 2015 at 21:12

ErikR

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Comments

WolfeFan · Accepted Answer · 2015-07-13 21:13:14Z

0

*Main> mapo (\(a,b) -> a+b) [(1,2)]
[3.0]

Your problem is that mapo expects a function of the tuple (Float,Float) but addition is a function on two Floats, rather than a tuple.

*Main> :t (+)
(+) :: Num a => a -> a -> a

You can use mapo as-is, as long as you remember to pass it a function which expects (Float,Float) and returns Float.

answered Jul 13, 2015 at 21:13

WolfeFan

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