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Iterating through a String and replacing single chars with substrings in haskell
I'm trying to implement a function that looks at a String ([Chars]) and checks for every letter whether this letter should be replaced with another string. For example we might have a [Chars] consisting of "XYF" and rules that says "X = HYHY", "Y = OO", then our output should become "HYHYOOF".
I want to use the following two types which I have defined:
type Letters = [Char]
data Rule = Rule Char Letters deriving Show
My idea is that the function should look something like the following below using guards. The problem is however I can't find any information on how to the recursive call should look like when i want browse through all my rules to see if any of them fits to the current letter x. I hope anyone can give some hints on how the notation goes.
apply :: Letters -> [Rule] -> Letters
apply _ _ = []
apply (x:xs) (Rule t r:rs)
| x /= t = apply x (Rule t rs)
| x == t = r++rs:x
| otherwise =
I would suggest a helper function to check whether a rule matches,
matches :: Char -> Rule -> Bool
matches c (Rule x _) = c == x
and then you check for each character whether there are any matching rules
apply :: Letters -> [Rule] -> Letters
apply [] _ = []
apply s [] = s
apply (c:cs) rules = case filter (matches c) rules of
[] -> c : apply cs rules
(Rule _ rs : _) -> rs ++ apply cs rules
If you try an explicit recursion on rules within apply, it will become too ugly, since you need to remember the full rules list for replacing later characters.