Question

I want to get the minimum values from two mappings and subtract one from the other. I’m really new to Haskell and am pretty embarrassed by my attempt but I wanted to give it a go before asking!

    calc :: (a -> b) -> [a] -> Float
    calc = a - b
         where
         a = minimum map1
         b = minimum map2
    map1 f xs = [f x | x <- xs]
    map2 f xs = [square x | x <- xs]
    square x = x*x

I’m getting so many errors that I feel like I must be doing it completely the wrong way?

Answer 1

The first problem is that map2 just discards the function it’s given. map1 is just the standard map function, so you don’t need to define it. map2 can be defined properly as:

map2 = map square

The second problem is that you haven’t supplied map1 and map2 with any arguments in your definition of calc. Is something like this what you want?

calc :: (a -> b) -> [a] -> Float
calc f xs = a - b
     where
     a = minimum (map f xs)
     b = minimum (map2 xs)

Basically, your problem is that you’re declaring parameters but not processing them, or using functions that take parameters without actually specifying them. That doesn’t work in any language 🙂

That’s not all, however. Your type for calc is wrong. I suggest you think about why this can’t work — in particular, what if I say a is String and b is ()? You can try removing the type signature of calc and entering :t calc into GHCi to find out what the type GHC infers for calc is to get a head-start on correcting it.