Real World Haskell

Making Use of Our Pattern Matcher

It’s all very well to have a function that can match glob patterns, but we’d like to be able to put this to practical use. On Unix-like systems, the glob function returns the names of all files and directories that match a given glob pattern. Let’s build a similar function in Haskell. Following the Haskell norm of descriptive naming, we’ll call our function namesMatching:

-- file: ch08/Glob.hs
module Glob (namesMatching) where

We specify that namesMatching is the only name that users of our Glob module will be able to see.

This function will obviously have to manipulate filesystem paths a lot, splicing and joining them as it goes. We’ll need to use a few previously unfamiliar modules along the way.

The System.Directory module provides standard functions for working with directories and their contents:

-- file: ch08/Glob.hs
import System.Directory (doesDirectoryExist, doesFileExist,
                         getCurrentDirectory, getDirectoryContents)

The System.FilePath module abstracts the details of an operating system’s path name conventions. The (</>) function joins two path components:

ghci> :m +System.FilePath
ghci> "foo" </> "bar"
Loading package filepath-1.1.0.0 ... linking ... done.
"foo/bar"

The name of the dropTrailingPathSeparator function is perfectly descriptive:

ghci> dropTrailingPathSeparator "foo/"
"foo"

The splitFileName function splits a path at the last slash:

ghci> splitFileName "foo/bar/Quux.hs"
("foo/bar/","Quux.hs")
ghci> splitFileName "zippity" ("","zippity") ...