GHC.Magic

ghc-prim-0.4.0.0: GHC primitives

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)
Safe Haskell | Trustworthy
Language | Haskell2010

GHC.Magic

Description

GHC magic.

Use GHC.Exts from the base package instead of importing this module directly.

Synopsis

- inline :: a -> a
- lazy :: a -> a
- oneShot :: (a -> b) -> a -> b

# Documentation

inline :: a -> a

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.

lazy :: a -> a

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 `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.

oneShot :: (a -> b) -> a -> b

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.