I am reading the book `The Haskell Road to Math, Logic, ...". One of
the exercises in the first chapter asks for a function that maps a
string "abcd" to "abbcccdddd" and "bang!" to "baannngggg!!!!!". Since
such a function f fixes the empty word, and maps wa to
f(w)a^(length(w)+1) for any word w and any letter a, I came up with the
following solution:
-- Map "abcd" to "abbcccdddd" and "bang!" to "baannngggg!!!!!".
blowup :: String -> String
blowup [] = []
blowup x = blowup (allButLast x) ++ lastToTheLength x
-- Map "abcd" to "abc".
allButLast :: String -> String
allButLast [] = []
allButLast [x] = []
allButLast (x : xs) = x : allButLast xs
-- Map "abcd" to d^4 = "dddd".
lastToTheLength :: String -> String
lastToTheLength [] = []
lastToTheLength [x] = [x]
lastToTheLength (_ : xs) = lastToTheLength xs ++ [last xs]
One question I have is whether I can eliminate points in the above
definition of blowup, and write something like
blowup = (++) . (blowup . allButLast, lastToTheLength)
thinking of (++) as a function String x String -> String. Also, I can't
figure out whether it is possible to get a shorter solution using fold.
I have tried Hlint on my file, but it gave no suggestions.
I am sure there are better ways, and would like some pointers and any
general suggestions for improvement.
Thanks and regards,
Raghavendra.
--
N. Raghavendra