Question

Hello I am trying to write a very simple function in Haskell. However I can’t get “ghci” to accept my code.

data Field = A1 Int deriving (Show)

data FieldList = FL [Field] | Name String deriving (Show)

t :: Field

t = A1 1

u :: Int -> FieldList

u 0 = FL []

u n = FL [t]:(u (n-1))

And the error I get is this:

test.hs:9:7:

Couldn't match expected type `FieldList' with actual type `[a0]'

    In the expression: (FL [t]) : (u (n - 1))
    In an equation for `u': u n = (FL [t]) : (u (n - 1))

Can someone point me in the right direction?

Thanks!

Answer 1

Looking at the last line:

u n = FL [t]:(u (n-1))

u has the type Int -> FieldList. n is an Int, so (n - 1) is also an Int. u (n-1) would therefor be a FieldList.

Function application has a higher precedence than operators, so the above line is equivalent to:

u n = (FL [t]) : (u (n - 1) )

FL [t] is a FieldList.

However, (:) has the type a -> [a] -> [a]. You can see the types don’t match, so that is what is causing the problem.

---

What you probably want to do is build up the list of Fields (having type [Field]), and then turning that into a FieldList. Here is some stub code:

u :: Int -> FieldList
u n = FL (uHelper n)

uHelper :: Int -> [Field]
uHelper = ... -- write this function