GHC.Magic

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.

The call noinline f arranges that f will not be inlined. It is removed during CorePrep so that its use imposes no overhead (besides the fact that it blocks inlining.)

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, Control.Parallel.par would look strict in y which would defeat the whole purpose of Control.Parallel.par.

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 representation-polymorphic: the type variables may refer to lifted or unlifted types.

Apply a function to a State# RealWorld token. When manually applying a function to realWorld#, it is necessary to use NOINLINE to prevent semantically undesirable floating. runRW# is inlined, but only very late in compilation after all floating is complete.

dataToTag# evaluates its argument and returns the index (starting at zero) of the constructor used to produce that argument. Any algebraic data type with all of its constructors in scope may be used with dataToTag#.

>>> dataToTag# (Left ())0#>>> dataToTag# (Right undefined)1#