CODEWITHSUNDEEP
listhaskell
Skyfe
Skyfe

Reputation: 581

haskell add item to list not working

I'm new to Haskell and I'm trying to add a string to a list of strings using the : operator, but somehow it's not working properly... This code is working:

sl:(printH header):sl:(map printR t)

but when I try to add the string sl on the end as well, like this:

sl:(printH header):sl:(map printR t):sl

it throws out an error, which doesn't make any sense to me (sincwe the other joins did go well):

Couldn't match type '[Char]' with 'Char'
Expected type: [String] -> Char
  Actual type: [String] -> String
In the first argument of 'map', namely 'printR'
...

Does anyone know why this happens?

Upvotes: 1

Views: 2634

Answers (2)

masonk
masonk

Reputation: 9778

Prelude> :type (:)
(:) :: a -> [a] -> [a]

This tells you that the (:) function takes a single item on the left and a list on the right. Given:

a :: t
b :: t
c :: [t],

a:b:c parses as a:(b:c), which works, because the expression b:c is of type [t]. Meanwhile, b:c:a parses as b:(c:a), which doesn't work, because c:a is ill-typed: a should be of type [t], but is actually of type t, and c should be of type t, but is actually of type [t]. In your example, t is Char.

If you want to add a single item to the end of a list, you can use concatenate (++):

xs ++ [i]

Upvotes: 5

nickie
nickie

Reputation: 5808

Operator : is used to prepend an element to a list. In your expression

sl:(printH header):sl:(map printR t)

the first three expressions (sl, printH header and sl) are list elements (strings, apparently), whereas the fourth one (map printR t) is the list to prepend those to.

If you want to append an element to the list, you cannot use operator :. You will have to use something like:

sl:(printH header):sl:(map printR t) ++ [sl]

Upvotes: 3

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