:: a -> a package:ihaskell

Identity function.

id 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 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.

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.)