Although popular CPUs allow program to control the rounding direction of floating-point operations, such feature is not directly accessible to Haskell.

Several options are available to control the rounding direction, including

• Emulate the operations using Rational.
• Emulate the desired rounding behavior using the default rounding direction (i.e. round to nearest).

• Provide the rounding-direction-controlled operations in C or assembly, and use FFI to call them from Haskell.

  • C FFI is portable, but has limitations (e.g. cannot return multiple values directly).
  • GHC-specific foreign import prim can return multiple values efficiently, but cannot be implemented in C.

This library implements the first and third options, in Numeric.Rounded.Hardware.Backend.ViaRational and Numeric.Rounded.Hardware.Backend.C/Numeric.Rounded.Hardware.Backend.FastFFI respectively.

The default implementation for Float and Double depends on the platform and package flags. To help the programmer identify which implementation is used, this module provides a function to obtain the name of implementation.

To disable use of FFI, enable the package flag pure-hs.