51 lines
3.3 KiB
Factor
51 lines
3.3 KiB
Factor
USING: help.markup help.syntax quotations kernel
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stack-checker.transforms sequences combinators ;
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IN: macros
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HELP: MACRO:
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{ $syntax "MACRO: word ( inputs... -- ) definition... ;" }
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{ $description "Defines a macro word. The definition must have stack effect " { $snippet "( inputs... -- quot )" } "." }
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{ $notes
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"A call of a macro inside a word definition is replaced with the quotation expansion at compile-time. The following two conditions must hold:"
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{ $list
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{ "All inputs to the macro call must be literals" }
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{ "The expansion quotation produced by the macro has a static stack effect" }
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}
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"Macros allow computation to be moved from run-time to compile-time, splicing the result of this computation into the generated quotation."
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}
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{ $examples
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"A macro that calls a quotation but preserves any values it consumes off the stack:"
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{ $code
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"USING: fry generalizations kernel macros stack-checker ;"
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"MACRO: preserving ( quot -- )"
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" [ inputs ] keep '[ _ ndup @ ] ;"
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}
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"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:"
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{ $code
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": ifte ( pred true false -- ) [ preserving ] 2dip if ; inline"
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}
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"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 " { $snippet "preserving" } " will expand at compile-time, and the generated machine code will be exactly the same as if the inputs consumed by the predicate were duplicated by hand."
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$nl
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"The " { $snippet "ifte" } " combinator presented here has similar semantics to the " { $snippet "ifte" } " combinator of the Joy programming language."
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} ;
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HELP: macro
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{ $class-description "Class of words defined with " { $link POSTPONE: MACRO: } "." } ;
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ARTICLE: "macros" "Macros"
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"The " { $vocab-link "macros" } " vocabulary implements " { $emphasis "macros" } ", which are code transformations that may run at compile-time under the right circumstances."
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$nl
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"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."
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$nl
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"Macros can also be used to calculate lookup tables and generate code at compile time, which can improve performance, raise the level of abstraction, and simplify code."
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$nl
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"Factor macros are similar to Lisp macros; they are not like C preprocessor macros."
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$nl
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"Defining new macros:"
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{ $subsections POSTPONE: MACRO: }
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"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."
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{ $subsections define-transform }
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{ $see-also "generalizations" "fry" } ;
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ABOUT: "macros"
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