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Neo Streets
Neo Streets

Reputation: 535

Using the replicate function in haskell to add to a list

I'm to use the replicate function that Haskell has (replicate 3 'x' = ['x','x','x']) but I'm trying to add it to a list that already exists.

Prelude > let k = ['d','f','g']
Prelude > (replicate 3 'd':k)

but I keep getting the error,

Couldn't match type 'Char' with '[Char]'
Expected type: [[Char]]
Actual type: [Char]

Is there a way to do this?

Upvotes: 1

Views: 211

Answers (1)

Benjamin Hodgson
Benjamin Hodgson

Reputation: 44634

k and replicate 3 'd' each have a type of [Char].

ghci> :t k
k :: [Char]

ghci> :t replicate 3 'd'
replicate 3 'd' :: [Char]

However, the type of (:) is a -> [a] -> [a]: it takes a single a and adds it to the list of as.

ghci> :t (:)
(:) :: a -> [a] -> [a]

So Haskell's giving you a type error because you can't apply (:) to two things of type [Char]. In other words, you can't put a [Char] inside a [Char].

What we really need is a function of type [a] -> [a] -> [a] which concatenates the two lists. It happens that this function is called (++).

ghci> :t (++)
(++) :: [a] -> [a] -> [a]

ghci> replicate 3 'd' ++ k
"ddddfg"

Upvotes: 4

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