GHC/Magic.hs

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude #-}
    {-# LANGUAGE MagicHash #-}
    {-# LANGUAGE UnboxedTuples #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Magic
    -- Copyright   :  (c) The University of Glasgow 2009
    -- License     :  see libraries/ghc-prim/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- GHC magic.
    --
    -- Use GHC.Exts from the base package instead of importing this
    -- module directly.
    --
    -----------------------------------------------------------------------------

    module GHC.Magic ( inline, lazy, oneShot, runRW# ) where

    import GHC.Prim
    import GHC.CString ()

    -- | The call @inline f@ arranges that 'f' is inlined, regardless of
    -- its size. More precisely, the call @inline f@ rewrites to the
    -- right-hand side of @f@'s definition. This allows the programmer to
    -- control inlining from a particular call site rather than the
    -- definition site of the function (c.f. 'INLINE' pragmas).
    --
    -- This inlining occurs regardless of the argument to the call or the
    -- size of @f@'s definition; it is unconditional. The main caveat is
    -- that @f@'s definition must be visible to the compiler; it is
    -- therefore recommended to mark the function with an 'INLINABLE'
    -- pragma at its definition so that GHC guarantees to record its
    -- unfolding regardless of size.
    --
    -- If no inlining takes place, the 'inline' function expands to the
    -- identity function in Phase zero, so its use imposes no overhead.
    {-# NOINLINE[0] inline #-}
    inline :: a -> a
    inline x = x

    -- | The 'lazy' function restrains strictness analysis a little. The
    -- call @lazy e@ means the same as 'e', but 'lazy' has a magical
    -- property so far as strictness analysis is concerned: it is lazy in
    -- its first argument, even though its semantics is strict. After
    -- strictness analysis has run, calls to 'lazy' are inlined to be the
    -- identity function.
    --
    -- This behaviour is occasionally useful when controlling evaluation
    -- order. Notably, 'lazy' is used in the library definition of
    -- 'Control.Parallel.par':
    --
    -- > par :: a -> b -> b
    -- > par x y = case (par# x) of _ -> lazy y
    --
    -- If 'lazy' were not lazy, 'par' would look strict in 'y' which
    -- would defeat the whole purpose of 'par'.
    --
    -- Like 'seq', the argument of 'lazy' can have an unboxed type.
    lazy :: a -> a
    lazy x = x
    -- Implementation note: its strictness and unfolding are over-ridden
    -- by the definition in MkId.lhs; in both cases to nothing at all.
    -- That way, 'lazy' does not get inlined, and the strictness analyser
    -- sees it as lazy.  Then the worker/wrapper phase inlines it.
    -- Result: happiness

    -- | The 'oneShot' function can be used to give a hint to the compiler that its
    -- argument will be called at most once, which may (or may not) enable certain
    -- optimizations. It can be useful to improve the performance of code in continuation
    -- passing style.
    --
    -- If 'oneShot' is used wrongly, then it may be that computations whose result
    -- that would otherwise be shared are re-evaluated every time they are used. Otherwise,
    -- the use of `oneShot` is safe.
    --
    -- 'oneShot' is open kinded, i.e. the type variables can refer to unlifted
    -- types as well.
    oneShot :: (a -> b) -> (a -> b)
    oneShot f = f
    -- Implementation note: This is wired in in MkId.lhs, so the code here is
    -- mostly there to have a place for the documentation.

    -- | Apply a function to a 'RealWorld' token.
    runRW# :: (State# RealWorld -> (# State# RealWorld, o #))
           -> (# State# RealWorld, o #)
    -- See Note [runRW magic] in MkId
    #if !defined(__HADDOCK_VERSION__)
    runRW# m = m realWorld#
    #else
    runRW# = runRW#   -- The realWorld# is too much for haddock
    #endif
    {-# NOINLINE runRW# #-}
    -- This is inlined manually in CorePrep