Having trouble with recursion and Type mismatching in Haskell

Question

-- genrep
 genrep :: a -> Int -> [a]
 genrep a n 
    | n == 0 = []
    |otherwise = a ++ genrep (a (n-1))

So I'm trying to make a simple replication function in haskell - one that would take a generic type a and replicate it n times. However, the above does not seem to work. I keep getting this error code:

*** Expression     : a ++ genrep (a (n - 1))
*** Term           : genrep (a (n - 1))
*** Type           : Int -> [b]
*** Does not match : [a]

Can anyone tell me what's going on? The function looks correct to me, but Haskell doesn't seem to like this.

OJ. · Accepted Answer · 2011-03-17 06:43:22Z

4

Change this:

| otherwise = a ++ genrep (a (n-1))

to this:

| otherwise = [a] ++ genrep a (n-1)

Your current version calls genrep recursively with only one argument when it expects two. The extra parenthesis aren't required.

Edit: I fixed up the above code to include the [a] instead of just a. Sorry about that. This is actually what I'd suggest you do:

genrep :: a -> Int -> [a]
genrep a 0 = []
genrep a n = a : genrep a (n-1)

edited Mar 17, 2011 at 6:43

answered Mar 17, 2011 at 6:20

OJ.

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2 Comments

Waffles Over a year ago

Hmm... that doesn't work. I get another error complaining about an infinite type unification with that.

Thomas M. DuBuisson Over a year ago

@Waffles [a] ++ is rather pointless, just use a :. The first makes a single element into a list then performs a concatenation of two lists, the second is just the list constructor to cons an element onto an existing list. OJ probably used the first form to closely match what you already had.

Ptival · Accepted Answer · 2011-03-17 11:28:39Z

2

You can also write it:

genrep :: a -> Int -> [a]
genrep a n = take n (repeat a)

Or even:

genrep = flip replicate

Since replicate exists: http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/GHC-List.html#replicate

If you don't know its name, you can find it by using this Hoogle search: http://www.haskell.org/hoogle/?hoogle=a+-%3E+Int+-%3E+%5Ba%5D

Usually you don't have to write this kind of recursion by hand, you can reuse functions like take or repeat.

answered Mar 17, 2011 at 11:28

Ptival

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