This module wraps the functions of the System.IO module at a different type namely SIO.

The purpose of this module is to export only strict IO functions, by strict we mean strict in the result. The arguments of these functions may only by partially forced, but when the function returns, the arguments can no longer be forced by the function. When the type of the value to be forced is polymorphic, a NFData constraint is added since we (internally) use rnf to force the value. Then we rely on the behavior of NFData instances to provide the fact that any lazy argument passed to a SIO function will not leak-out after the call.

These functions do not necessarily use their arguments completely but they do not hold or return any value that could depend on these arguments. If the original functions from System.IO module were already strict, then this module just provides them at another type. Some functions from the original module are famously lazy like the getContents like functions: in this module their results are deeply forced.

In Haskell, monad operations (return and >>=) have to be lazy. Therefore the SIO monad is not completely strict (i.e. pure values can still be lazy). So in this module we expose the return' function that forces the given value before putting it into the monad.

Since this module uses the same names as System.IO, it is designed to be imported qualified.

    import System.IO.Strict (SIO)
    import qualified System.IO.Strict as SIO