This module provides an overloaded function, deepseq, for fully evaluating data structures (that is, evaluating to "Normal Form").

A typical use is to prevent resource leaks in lazy IO programs, by forcing all characters from a file to be read. For example:

 import System.IO
 import Control.DeepSeq
 
 main = do
     h <- openFile "f" ReadMode
     s <- hGetContents h
     s `deepseq` hClose h
     return s

deepseq differs from seq as it traverses data structures deeply, for example, seq will evaluate only to the first constructor in the list:

 > [1,2,undefined] `seq` 3
 3

While deepseq will force evaluation of all the list elements:

 > [1,2,undefined] `deepseq` 3
 *** Exception: Prelude.undefined

Another common use is to ensure any exceptions hidden within lazy fields of a data structure do not leak outside the scope of the exception handler, or to force evaluation of a data structure in one thread, before passing to another thread (preventing work moving to the wrong threads).