"Using this macro, we can define a variant of " { $link if } " which takes a predicate quotation instead of a boolean; any values consumed by the predicate quotation are restored immediately after:"
{ $code
": ifte ( pred true false -- ) [ preserving ] 2dip if ; inline"
}
"Note that " { $snippet "ifte" } " is an ordinary word, and it passes one of its inputs to the macro. If another word calls " { $snippet "ifte" } " with all three input quotations literal, then " { $snippet "ifte" } " will be inlined and " { $snippet "preserving" } " will expand at compile-time, and the generated machine code will be exactly the same as if the inputs consumed by the predicate were duplicated by hand."
$nl
"The " { $snippet "ifte" } " combinator presented here has similar semantics to the " { $snippet "ifte" } " combinator of the Joy programming language."
"The " { $vocab-link "macros" } " vocabulary implements " { $emphasis "macros" } ", which are code transformations that may run at compile-time under the right circumstances."
"Macros can be used to implement combinators whose stack effects depend on an input parameter. Since macros are expanded at compile time, this permits the compiler to infer a static stack effect for the word calling the macro."
$nl
"Macros can also be used to calculate lookup tables and generate code at compile time, which can improve performance, raise the level of abstraction, and simplify code."
"A slightly lower-level facility, " { $emphasis "compiler transforms" } ", allows an ordinary word definition to co-exist with a version that performs compile-time expansion. The ordinary definition is only used from code compiled with the non-optimizing compiler. Under normal circumstances, macros should be used instead of compiler transforms; compiler transforms are only used for words such as " { $link cond } " which are frequently invoked during the bootstrap process, and this having a performant non-optimized definition which does not generate code on the fly is important."
