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diff --git a/README.md b/README.md new file mode 100644 index 000000000000..2d590a0564ae --- /dev/null +++ b/README.md @@ -0,0 +1,258 @@ +quasiquote-2.0 +============== + +Why should it be hard to write macros that write other macros? +Well, it shouldn't! + +quasiquote-2.0 defines slightly different rules for quasiquotation, +that make writing macro-writing macros very smooth experience. + +NOTE: quasiquote-2.0 does horrible things to shared structure!!! +(it does a lot of COPY-TREE's, so shared-ness is destroyed). +So, it's indeed a tool to construct code (where it does not matter much if the +structure is shared or not) and not the data (or, at least, not the data with shared structure) + + +```lisp +(quasiquote-2.0:enable-quasiquote-2.0) + +(defmacro define-my-macro (name args &body body) + `(defmacro ,name ,args + `(sample-thing-to-expand-to + ,,@body))) ; note the difference from usual way + +(define-my-macro foo (x y) + ,x ; now here injections of quotation constructs work + ,y) + +(define-my-macro bar (&body body) + ,@body) ; splicing is also easy +``` + +The "injections" in macros FOO and BAR work as naively expected, as if I had written +```lisp +(defmacro foo (x y) + `(sample-thing-to-expand-to ,x ,y)) + +(defmacro bar (&body body) + `(sample-thing-to-expand-to ,@body)) + +(macroexpand-1 '(foo a b)) + + '(SAMPLE-THING-TO-EXPAND-TO A B) + +(macroexpand-1 '(bar a b c)) + + '(SAMPLE-THING-TO-EXPAND-TO A B C) +``` + + +So, how is this effect achieved? + + +DIG, INJECT and SPLICE +------------------------- + +The transformations of backquote occur at macroexpansion-time and not at read-time. +It is totally possible not to use any special reader syntax, but just +underlying macros directly! + +At the core is a macro DIG, which expands to the code that generates the +expression according to the rules, which are roughly these: + * each DIG increases "depth" by one (hence the name) + * each INJECT or SPLICE decreases "depth" by one + * if depth is 0, evaluation is turned on + * if depth if not zero (even if it's negative!) evaluation is off + * SPLICE splices the form, similarly to ordinary `,@`, INJECT simply injects, same as `,` + +```lisp +;; The example using macros, without special reader syntax + +(dig ; depth is 1 here + (a b + (dig ; depth is 2 here + ((inject c) ; this inject is not evaluated, because depth is nonzero + (inject (d ;depth becomes 1 here again + (inject e) ; and this inject is evaluated, because depth becomes zero + )) + (inject 2 f) ; this inject with level specification is evaluated, because it + ; decreases depth by 2 + )))) + + +;; the same example using ENABLE-QUASIQUOTE-2.0 syntax is written as +`(a b `(,c ,(d ,e) ,,f)) ; note double comma acts different than usually +``` + + +The ENABLE-QUASIQUOTE-2.0 macro just installs reader that reads +`FORM as (DIG FORM), ,FORM as (INJECT FORM) and ,@FORM as (SPLICE FORM). +You can just as well type DIG's, INJECT's and SPLICE's directly, +(in particular, when writing utility functions that generate macro-generating code) +or roll your own convenient reader syntax (pull requests are welcome). + +So, these two lines (with ENABLE-QUASIQUOTE-2.0) read the same +```lisp +`(a (,b `,,c) d) + +(dig (a ((inject b) (dig (inject 2 c))) d)) +``` + +You may notice the (INJECT 2 ...) form appearing, which is described below. + + +At "level 1", i.e. when only \` , and ,@ are used, and not, say \`\` ,, ,', ,,@ ,',@ +this behaves exactly as usual quasiquotation. + + +The optional N argument +-------------- + +All quasiquote-2.0 operators accept optional "depth" argument, +which goes before the form for human readability. + +Namely, (DIG N FORM) increases depth by N instead of one and +(INJECT N FORM) decreases depth by N instead of one. + +```lisp +(DIG 2 (INJECT 2 A)) + +; gives the same result as + +(DIG (INJECT A)) +``` + + +In fact, with ENABLE-QUASIQUOTE-2.0, say, ,,,,,FORM (5 quotes) reads as (INJECT 5 FORM) +and ,,,,,@FORM as (SPLICE 5 FORM) + + +More examples +------------- + +For fairly complicated example, which uses ,,,@ and OINJECT (see below), + see DEFINE-BINOP-DEFINER macro +in CG-LLVM (https://github.com/mabragor/cg-llvm/src/basics.lisp), +desire to write which was the initial impulse for this project. + + +For macro, that is not a macro-writing macro, yet benefits from +ability to inject using `,` and `,@`, consider JOINING-WITH-COMMA-SPACE macro +(also from CG-LLVM) + +```lisp +(defmacro joining-with-comma-space (&body body) + ;; joinl just joins strings in the list with specified string + `(joinl ", " (mapcar #'emit-text-repr + (remove-if-not #'identity `(,,@body))))) + +;; the macro can be then used uniformly over strings and lists of strings +(defun foo (x y &rest z) + (joining-with-comma-space ,x ,y ,@z)) + +(foo "a" "b" "c" "d") + ;; produces + "a, b, c, d" +``` + + +ODIG and OINJECT and OSPLICE +---------------------------- + +Sometimes you don't want DIG's macroexpansion to look further into the structure of +some INJECT or SPLICE or DIG in its subform, +if the depth does not match. In these cases you need "opaque" versions of +DIG, INJECT and SPLICE, named, respectively, ODIG, OINJECT and OSPLICE. + +```lisp +;; here injection of B would occur +(defun foo (b) + (dig (dig (inject (a (inject b)))))) + +;; and here not, because macroexpansion does not look into OINJECT form +(defun bar (b) + (dig (dig (oinject (a (inject b)))))) + +(foo 1) + + '(DIG (INJECT (A 1))) + +(bar 1) + + '(DIG (OINJECT (A (INJECT B)))) +``` + +MACRO-INJECT and MACRO-SPLICE +----------------------------- + +Sometimes you just want to abstract-out some common injection patterns... +That is, you want macros, that expand into common injection patterns. +However, you want this only sometimes, and only in special circumstances. +So it won't do, if INJECT and SPLICE just expanded something, whenever it +turned out to be macro. For that, use MACRO-INJECT and MACRO-SPLICE. + +```lisp +;; with quasiquote-2.0 syntax turned on +(defmacro inject-n-times (form n) + (make-list n :initial-element `(inject ,form))) + +(let (x 0) + `(dig (a (macro-inject (inject-n-times (incf x) 3))))) +;; yields +'(a (1 2 3)) + +;;and same with MACRO-SPLICE +(let (x 0) + `(dig (a (macro-splice (inject-n-times (incf x) 3))))) +;; yields +'(a 1 2 3) +``` + +OMACRO-INJECT and OMACRO-SPLICE are, as usual, opaque variants of MACRO-INJECT and MACRO-SPLICE. + +Both MACRO-INJECT and MACRO-SPLICE expand their subform exactly once (using MACROEXPAND-1), +before plugging it into list. +If you want to expand as much as it's possible, use MACRO-INJECT-ALL and MACRO-SPLICE-ALL, +which expand using MACROEXPAND before injecting/splicing, respectively. +That implies, that while subform of MACRO-INJECT and MACRO-SPLICE is checked to be +macro-form, the subform of MACRO-INJECT-ALL is not. + + +Terse syntax of the ENABLE-QUASIQUOTE-2.0 +----------------------------------------- + +Of course, typing all those MACRO-INJECT-ALL, or OMACRO-SPLICE-ALL or whatever explicitly +every time you want this special things is kind of clumsy. For that, default reader +of quasiquote-2.0 provides extended syntax + +```lisp +',,,,!oma@x + +;; reads as +'(OMACRO-SPLICE-ALL 4 X) +``` + +That is, the regexp of the syntax is +[,]+![o][m][a][@]<whatever> + +As usual, number of commas determine the anti-depth of the injector, exclamation mark +turns on the syntax, if `o` is present, opaque version of injector will be used, +if `m` is present, macro-expanding version of injector will be used and if +`a` is present, macro-all version of injector will be used. + +Note: it's possible to write ,!ax, which will read as (INJECT-ALL X), but +this will not correspond to the actual macro name. + +Note: it was necessary to introduce special escape-char for extended syntax, +since usual idioms like `,args` would otherwise be completely screwed. + + +TODO +---- + +* WITH-QUASIQUOTE-2.0 read-macro-token for local enabling of ` and , overloading +* wrappers for convenient definition of custom overloading schemes +* some syntax for opaque operations + +P.S. Name "quasiquote-2.0" comes from "patronus 2.0" spell from www.hpmor.com + and has nothing to do with being "the 2.0" version of quasiquote. \ No newline at end of file |