Haskell: Using map with a function that returns a list?

Question

I have this encode function:

class Encode a where
  encode :: a -> [Bit]

and I have problems writing a function that encodes a list of type a to a list of bits. I want to recursively encode the elements of a list. In my understanding you can use the map function for that purpose. The problem is that encode returns a list [Bit], whereas map expects just Bit. How can I solve this? Here is the relevant part of the program.

instance Encode a => Encode [a] where
    encode [] = [I, O, O, I, O, I]
    encode m = ([I, O, O] ++ (map encode m) ++ [I, O, I])

dave4420 · Accepted Answer · 2013-05-09 18:08:26Z

6

Use concatMap. It concatenates the results after mapping them.

instance Encode a => Encode [a] where
    encode [] = [I, O, O, I, O, I]
    encode m = ([I, O, O] ++ (concatMap encode m) ++ [I, O, I])

How you could have found this out for yourself: if you search for the type of the function you want, (a -> [Bit]) -> [a] -> [Bit], using Hoogle, concatMap is the first result.

answered May 9, 2013 at 18:08

dave4420

47.2k6 gold badges121 silver badges153 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Daniel Fischer Over a year ago

You don't even need to special-case the empty list, @user2367390, it's also covered by the second equation.

isturdy Over a year ago

Following up on the comment about Hoogle--a useful approach to such situations is to put a custom function in place, define it with undefined, and then use ghci to find the type (:t function). That shows you the type the compiler infers; that can help writing the function, or you can search for the signature with Hoogle. (Here that would mean defining encode m = ([I, O, O] ++ (map' encode m) ++ [I, O, I]) and map' = undefined; loading in ghci and typing :t map').

Collectives™ on Stack Overflow

Haskell: Using map with a function that returns a list?

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related