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6.9.9 Inlinable Procedures

You can define an inlinable procedure by using define-inlinable instead of define. An inlinable procedure behaves the same as a regular procedure, but direct calls will result in the procedure body being inlined into the caller.

Bear in mind that starting from version 2.0.3, Guile has a partial evaluator that can inline the body of inner procedures when deemed appropriate:

scheme@(guile-user)> ,optimize (define (foo x)
                                 (define (bar) (+ x 3))
                                 (* (bar) 2))
$1 = (define foo
       (lambda (#{x 94}#) (* (+ #{x 94}# 3) 2)))

The partial evaluator does not inline top-level bindings, though, so this is a situation where you may find it interesting to use define-inlinable.

Procedures defined with define-inlinable are always inlined, at all direct call sites. This eliminates function call overhead at the expense of an increase in code size. Additionally, the caller will not transparently use the new definition if the inline procedure is redefined. It is not possible to trace an inlined procedures or install a breakpoint in it (see Traps). For these reasons, you should not make a procedure inlinable unless it demonstrably improves performance in a crucial way.

In general, only small procedures should be considered for inlining, as making large procedures inlinable will probably result in an increase in code size. Additionally, the elimination of the call overhead rarely matters for large procedures.

Scheme Syntax: define-inlinable (name parameter …) body1 body2 …

Define name as a procedure with parameters parameters and bodies body1, body2, ....