Hi,<br><br>I am new to functional and lazy programming languages ( that's correct, my life has been pretty pathetic so far) and am not able to understand GHC's behaviour for a particular function. Can someone help me please?
<br><br>I am trying to write a function which would compare two strings (from reverse) and return the position of first mismatch. This is part of the right-to-left scan of bayer-moore algorithm.. and it is essential for me to do it from reverse.
<br>Since my goal is to learn haskell, I am not using Data.ByteString.<br><br>My function is as follows:<br><br>matchReverse :: String -> String ->Int->(Bool,Int)<br>matchReverse   pos = (True,pos)<br>matchReverse _  pos = (False,pos)
<br>matchReverse  _ pos = (False,pos)<br>matchReverse (x:xs) (y:ys) pos = let (matched,pos) = matchReverse xs ys (pos +1)<br> in if matched then ((x==y),pos) <br> else (False,pos)
<br><br><br><br>The behaviour I expected in four scenarios is as below:<br>1.matchReverse "kapilash" "kapilash" 0 --should return (True,0)<br>2.matchReverse "kapilash" "kapilast" 0 --should return (False,8)
<br>3.matchReverse str1 str2 0 --should return (False,0)<br>4.matchReverse str1 str1 0 --should return (True,0)<br><br>where str1 and str2 are defined as below:<br> str1 = replicate 1000 'a'
<br> str2 = 'b':(replicate 999 'a')<br><br>what confounds me is that it is able to identify the first element of the tuple in ALL the cases.<br>Invoking fst on the each of the four calls instantly returns the expected value.(even for the cases 3 and 4 where, there are thousand elements)
<br>But it seems to go into an infinite loop while calculating the 'snd' of the tuple. Even for strings containing just one element each.<br>can someone throw some light on this please? Why does it go into an infinite loop?
<br><br>Many thanks<br>Kapilash<br><br><br>-- <br>I drink I am thunk.