Pattern matching string prefixes in Haskell

Question

Let's say I want to make a special case for a function that matches strings that start with the character 'Z'. I could easily do it using pattern matching by doing something like the following:

myfunc ('Z' : restOfString) = -- do something special myfunc s = -- do the default case here 

But what if I want to match strings with a longer prefix? Say I want to have a special case for strings that start with the word "toaster". What's the best way to write a pattern to match such a string?

Answer 1

myfunc ('t':'o':'a':'s':'t':'e':'r' : restOfString) = ... 

Using a normal pattern match works, but gets bothersome as the prefix string gets longer.

{-# LANGUAGE PatternGuards #-} import Data.List myFunc string | Just restOfString <- stripPrefix "toaster" string =     -- do something special myFunc string = -- do the default case here 

Using a library function instead of a pattern match is a bit easier to read and write.

{-# LANGUAGE ViewPatterns #-} import Data.List myFunc (stripPrefix "toaster" -> Just restOfString) = -- do something special myFunc string = -- do the default case here 

A GHC 6.10 syntax extension makes this usage even more natural.

---

Of course, the latter two are completely equivalent, and we can make do (messily) without any sugar at all.

import Data.List myFunc string =     if restIsJust       then -- do something special       else -- do the default case here   where     (restIsJust, restOfString) =         case stripPrefix "toaster" string of             Just something -> (True, something)             Nothing -> (False, undefined) 

Those syntax extensions are meant to make life easier for us, though.

Answer 2

import Data.List  myFunc str | "toaster" `isPrefixOf` str = something restOfString            | otherwise = somethingElse     where Just restOfString = stripPrefix "toaster" str