[Haskell-cafe] Re: subsequence

Peter Stranney peterstranney at yahoo.co.uk
Sun Oct 17 09:55:07 EDT 2004

>> I am stuck on an exercise in a haskell book (the craft of functional
>> programming) chapter 7 exercise 7.18 I have to write a function to 
>> does one list occur as a subsequence of another, i am having real
>> difficulty trying to do this. I have a method which will tell me if 
>> character occurs in the list but it won't work for a list of 
>> the function should look like isSubsequence :: [chars] -> [chars] ->
>> boolean

>Try writing a list or string equality function first? That may help.

Thanks for getting back to me, have written this so far
find :: [Char] -> Char -> Bool
find [] n = False
find (x:xs) n
   | x == n = True
   | otherwise = find xs x
this will work to find if one character appears in a list but i dont understand how to call this recursively to find if a list occurs within a list. I have tried writing another function to call this but it won't work. Does anyone have a solution?
thanks again

Peter Stranney

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