Haskell Gem – Unwrapping Indented Text

While processing some email, I needed a script to unwrap indented lines in email headers. I wrote a Haskell program that turned out to be a short and sweet demonstration of simple but interesting Haskell features (pattern matching, guards, type inference, function composition, and the humble cons) so I thought I’d share it.

If you’re new to Haskell,

• [] is the empty list
• [a] is a list with a single element a
• x : y creates a new list by placing element x at the front of list y
• ++ is concatenate
• interact is a function taking a single argument, a function that takes a string and returns a string, and returns a bit of IO that runs the string transformation function on the contents of STDIN and prints the result
• . (period) is a binary operator that composes functions
• foldr is right-fold, and behaves like this: foldr f d [a, b, c] == f a (f b (f c d))
• Lines of the form | a = b are guards, which evaluate to b if a evaluates to True

Example usage:

[/tmp]% cat > header
This is a normal line.
This is a slightly longer line that
__wraps with two spaces.
A short line.
Another example of a long line that
____wraps with tabs (not just once,
____but twice.
Final line.
[/tmp]% cat header | runhaskell Unwrap.hs
This is a normal line.
This is a slightly longer line that wraps with two spaces.
A short line.
Another example of a long line that wraps with tabs (not just once, but twice.
Final line.