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)

As far as I am aware, there is no more succinct syntax than that.

You can of course also just check whether the string starts with toaster in a guard-clause or an if inside the function body.

Answer 2

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 3

import Data.List

myFunc str | "toaster" `isPrefixOf` str = something restOfString
           | otherwise = somethingElse
    where Just restOfString = stripPrefix "toaster" str

Answer 4

The Split library, http://hackage.haskell.org/packages/archive/split/0.1.1/doc/html/Data-List-Split.html has many functions for splitting strings with strings, including prefix matching. You might find something useful there.

Answer 5

Offhand it looks pretty straightforward to write and invoke Template Haskell to achieve the desired effect. I'm not enough of a TH person to be sure, though.

Answer 6

myFunc str =
  case stripPrefix "toaster" str of
     Just restOfString -> something restOfString
     Nothing -> somethingElse

This is why stripPrefix returns a Maybe type.