Haskell Lists of Maybe Int's, convert each element to char, then String

Question

Need help with a haskell problem thats doing my head in. I know the steps (sorta) that need to be taken to achieve want I want but I don't really know how to go about doing it.

E.g

Say I have a lists of Maybe Int's - [[Just 2, Nothing, Just 3],[Just 6,Just 3, Just 3],[Nothing,Nothing, Just 1]]

I need to create a function that.

1) Takes lists of Maybe Ints

[[Just 2, Nothing, Just 3],[Just 6,Just 3, Just 3],[Nothing,Nothing, Just 1]]

2) Gets First List (head?)

[Just 2, Nothing, Just 3]

3) Recurse through each element within the list - (x:xs)

Just 2, Nothing, Just 3

4) convert Maybe Int to a Char - fromEnum

'2', '   ', '3'

5) return the complete String containing all the chars - (++) = [char] / String

"2   3"

6) move onto the next List until list = []

[Just 6,Just 3, Just 3]

So the end result would be something like this printed out on a separate line:

"2   3"
"633"
"     1"

I tried to explain it as well as possible and any tips / helps / sources of information would be appreciated.

Just FYI, you keep saying you want to operate on a list of Maybe Ints, but your examples each have Lists of List Maybe Int. Ie [[Maybe Int]] — Sarah
– Sarah, Commented Mar 15, 2012 at 9:46

hammar · Accepted Answer · 2012-03-15 09:40:20Z

9

You can use the maybe function, which allows you apply a function to the value in a Just with a default if it was Nothing, so to convert a Maybe Int to a single-digit Char with space as the default, use maybe ' ' intToDigit. (toEnum would use the ASCII value, which is not what you want here).

Then, to apply that within a list of lists, you just use map twice:

> import Data.Char
> map (map $ maybe ' ' intToDigit) [[Just 2, Nothing, Just 3],[Just 6,Just 3, Just 3], [Nothing, Nothing, Just 1]]
["2 3","633","  1"]

Since a string is just a list of characters, the result is a list of strings.

answered Mar 15, 2012 at 9:40

hammar

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1 Comment

danieltahara Over a year ago

After you're done, you print it with unlines. So the whole thing should be unlines. map (map $ maybe ' ' intToDigit) $ [your list]

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