PercentFormat – C-like printf-style string formatting for Haskell

The Text.PercentFormat library provides printf-style string formatting. It provides a % operator (as in Ruby or Python) and uses the old C-printf-style format you know and love.

This library differs from Text.Printf in that it does not rely on custom typeclasses – it works on anything that is a Show instance.

Formatting one value:

> import Text.PercentFormat
> "Hello %s!" -% "World"
"Hello World!"

Formatting three values, tuple style:

> "load average: %1.2f %1.2f %1.2f" -%%% (0.00, 0.066, 0.11)
"load average: 0.00 0.07 0.11"

Formatting three values, chain style:

> "load average: %1.2f %1.2f %1.2f" % 0.00 % 0.066 -% 0.11
"load average: 0.00 0.07 0.11"

To produce a string with a percent sign (%), use two percent signs (%%):

> "memory usage: %i%%" -% 13
"memory usage: 13%"

Percent signs are duplicated when using the % operator to allow chaining:

> "percent sign: %s, memory usage: %i%%" % "%" % 87
"percent sign: %%, memory usage: 87%%"

Always use the -% operator when formatting the last value to remove duplicate % signs:

> "percent sign: %s, memory usage: %i%%" % "%" -% 87
"percent sign: %, memory usage: 87%"

To print, just prefix you format expression with putStrLn $:

> putStrLn $ "Hello %s!" -% "World"
Hello World!