I can has(kell) moar cheezburgers?

KITTEH HAS IMPROOVD CODE SKILLS, AN NAO HAS BETTR CODE 4 U

This is an improvement of my original Haskell Lolcats Translator, and now features a function to do the translation (recursively?) using a map. I still have two let statements, which I know are frowned upon, but I couldn't get both the strToUpper and inline fmap working, so I opted for strToUpper.

Sample IO:

Enter your text to translate:
Let me have some cake on a plate please
LEMME HAS SUM CAEK ON PLAET PLZ

Teh Codez

import Data.Map
import Data.Strings

type Dictionary = [(String, String)]
lolcatDictionary =
    [
        (" A ", " "),
        ("CAN I", "I CAN"),
        ("MORE", "MOAR"),
        ("CHEESEBURGER", "CHEEZBURGER"),
        ("HAVE", "HAS"),
        ("HI", "HAI"),
        ("GHOST", "GOAST"),
        ("FEET", "FEAT"),
        ("MOAN", "MOWN"),
        ("CROWD", "CROUD"),
        ("NOTHING", "NUTHING"),
        ("MY", "MAI"),
        ("THEM", "DEM"),
        ("THESE", "THEES"),
        ("YOU", "YU"),
        ("LET ME", "LEMME"),
        ("KITE", "KIET"),
        ("CAT", "KAT"),
        ("CATS", "KATS"),
        ("KITTEN", "KITTEH"),
        ("KITTY", "KITTEH"),
        ("KITTENS", "KITTEHS"),
        ("LIKE", "LIEK"),
        ("COME", "COEM"),
        ("CAME", "CAEM"),
        ("BAKE", "BAEK"),
        ("PLATE", "PLAET"),
        ("SOME", "SUM"),
        ("CAKE", "CAEK"),
        ("PLEASE", "PLZ")
    ]

main :: IO ()
main = do
    putStrLn "Enter your text to translate:"
    input  String -> String
stringMapReplace [] string = string
stringMapReplace (m:ms) s = stringMapReplace ms (strReplace (fst m) (snd m) s)

What is the Data.Map doing there? It looks like you were going to use it, but then changed your mind and used a list of pairs instead.

The stringMapReplace function is doing explicit recursion. A more advanced Haskeller would recognize this pattern as some kind of fold:

-- if the list is empty, the result is the initial value z; else
-- apply f to the first element and the result of folding the rest
foldr f z []     = z 
foldr f z (x:xs) = f x (foldr f z xs) 

-- if the list is empty, the result is the initial value; else
-- we recurse immediately, making the new initial value the result
-- of combining the old initial value with the first element.
foldl f z []     = z                  
foldl f z (x:xs) = foldl f (f z x) xs

In this case, the function can be tersely written as

stringMapReplace :: Dictionary -> String -> String
stringMapReplace = flip $ foldr $ uncurry strReplace

Intermediate steps to get there

First, recognize that your dictionary is the thing that is being iterated through. (That is, your (m:ms) plays the role of the (x:xs) in the fold.) In Haskell, it is usually more convenient to place the parameter that varies the most at the end. So, rewrite your original

stringMapReplace (m:ms) s = stringMapReplace ms (strReplace (fst m) (snd m) s)

as

stringMapReplace :: Dictionary -> String -> String
stringMapReplace map s = stringMapReplace' s map
  where
    stringMapReplace' s [] = s
    stringMapReplace' s (m:ms) = stringMapReplace' (strReplace (fst m) (snd m) s) ms

Now, we can see that stringMapReplace' fits the pattern for foldl. However, foldr is preferable to foldl. If we perform the substitutions in a different order…

stringMapReplace :: Dictionary -> String -> String
stringMapReplace map s = stringMapReplace' s map
  where
    stringMapReplace' s [] = s
    stringMapReplace' s (m:ms) = strReplace (fst m) (snd m) (stringMapReplace' s ms)

… we can make stringMapReplace' fit the pattern for foldr.

stringMapReplace map s = stringMapReplace' s map
  where
    stringMapReplace' s map = foldr f s map
    f m = strReplace (fst m) (snd m)

Notice that in stringMapReplace' s map = foldr f s map, the s map is repeated on both sides. We can write that in point-free style as stringMapReplace' = foldr f.

But what is f? We can simplify it as uncurry strReplace.

stringMapReplace map s = stringMapReplace' s map
  where
    stringMapReplace' = foldr $ uncurry strReplace

What is the relationship between stringMapReplace and stringMapReplace'? The parameters are flipped.

stringMapReplace = flip $ foldr $ uncurry strReplace

QED.

StackExchange Code Review Q#131859, answer score: 2