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* Argot: A System For Grammar-Driven Macros
The ~argot~ system provides a macro-defining-macro called
~deflanguage~, which accepts a context-free grammar that is used to
parse the body calls to the defined macro.
** A brief tour
In ~/examples/calc.lisp~ you see the following ~deflanguage~:
#+begin_src lisp
(deflanguage calc (:documentation "A calculator language")
(<start>
:match (:or
(:seq <subexpr> (:eof))
(:seq <value> (:eof))
(:seq <unop> (:eof))
(:seq <binop> (:eof)))
:then car)
(<expr>
:match (:or <subexpr> <value> <unop> <binop>))
(<subexpr>
:match (:{} calc))
(<value>
:match (:item)
:if numberp)
(<binop>
:match (:seq
(:@ lhs <expr>)
(:@ rhs (:+ (:seq (:or= + - / * ^ %) <expr>))))
:then (expand-binop lhs rhs))
(<unop>
:match (:seq (:or= sin cos tan -) <expr>)))
#+end_src
This defines a cacluator language that you can use like so:
#+begin_src lisp
> (calc 1)
1
> (calc 1 + 2 + 3)
6
> (calc 1 + 2 * 3)
7
> (calc (1 + 2) * 3)
9
> (calc (1 + 2) * 3 + 1)
10
> (calc (1 + 2) * (3 + 1))
12
> (calc 2 ^ (2 * (1 + 1)))
16
#+end_src
The symbol ~calc~ also has a function docstring. If you are using
slime, you can evoke ~M-x slime-documentation~ with your cursor over
the ~calc~ symbol to see this:
#+begin_src
Documentation for the symbol CALC:
Function:
Arglist: (&BODY TOKENS)
A calculator language
<START> ::= (<SUBEXPR> ::EOF:: | <VALUE> ::EOF:: | <UNOP> ::EOF:: | <BINOP> ::EOF::)
<EXPR> ::= (<SUBEXPR> | <VALUE> | <UNOP> | <BINOP>)
<SUBEXPR> ::= {CALC}
<VALUE> ::= ::TOKEN::
<BINOP> ::= <EXPR> ('+' | '-' | '/' | '*' | '^' | '%') <EXPR>+
<UNOP> ::= ('SIN' | 'COS' | 'TAN' | '-') <EXPR>
KEY:
::TOKEN:: Any ole token
::EOF:: Explicitly match the end of the input
{GRAMMAR} Parse a sublist of tokens with GRAMMAR
(a|b|..) One of the alternavites a b ...
PATTERN+ One or more PATTERN
PATTERN* Zero or more PATTERN
<RULE> A nonterminal symbol - naming a parse rule
[OPT] Zero or one of OPT
#+end_src
** User Guide
The body of a ~deflanguage~ looks like this:
~(deflanguage NAME (&key (DOCUMENTATION "")) START-RULE &body RULES)~
~NAME~ is a symbol. This symbol becomes the name the macro
you're defining.
~START-RULE~ and ~RULES~ are *rule definition forms*, each of which
looks like this:
~(NONTERMINAL :match PATTERN [:if PREDICATE] [:then ACTION])~
A nonterminal is a symbol whose name is surrounded by angle brackets:
e.g. ~<RULE1>~ or ~<FOOBAR>~.
*** Pattern Expressions
There are two kinds of patterns. First, any nonterminal counts as a
valid pattern. Every other pattern is a list whose ~CAR~ is a keyword
and whose ~CDR~ varies depending on the value of the ~CAR~.
| PATTERN | MATCHES |
|----------------------+--------------------------------------------------|
| ~(:seq . PATTERNS)~ | Matches a sequence of patterns, results |
| | in the sequence of results. |
|----------------------+--------------------------------------------------|
| ~(:? PATTERN)~ | Optional match of PATTERN. Always succeeds. |
| | Succeeds with NIL if PATTERN doesn't match. |
|----------------------+--------------------------------------------------|
| ~(:* PATTERN)~ | Zero or more of PATTERN, results in sequence |
| | of matches. Always succeeds. |
|----------------------+--------------------------------------------------|
| ~(:+ PATTERN)~ | One ore more of PATTERN, results in a sequence. |
|----------------------+--------------------------------------------------|
| ~(:or . PATTERNS)~ | Matches one of PATTERNS, checked left to right. |
|----------------------+--------------------------------------------------|
| ~(:= . LITERAL)~ | Literal pattern matches. They match exactly |
| ~(:seq= . LITERALS)~ | their arguments (according to EQUALP). These |
| ~(:?= LITERAL)~ | variants behave like their counterparts above, |
| ~(:*= LITERAL)~ | except with literal value matches instead of |
| ~(:+= LITERAL)~ | pattern expressions. |
| ~(:or= . LITERALS)~ | |
|----------------------+--------------------------------------------------|
| ~(:@ VAR PATTERN)~ | Variable Binding. Matches PATTERN and binds the |
| | result to VAR, which is in-scope for the body of |
| | :IF and :THEN clauses (see below). |
|----------------------+--------------------------------------------------|
| ~(:{} LANGUAGE)~ | Match a list of TOKENS using a grammar named |
| | by LANGUAGE. I.e this lets you compose languages |
| | defined with DEFLANGUAGE. This is also the only |
| | way to parse sublists in the TOKENS list. |
|----------------------+--------------------------------------------------|
| ~(:item)~ | Matches any token in TOKENS. |
|----------------------+--------------------------------------------------|
| ~(:eof)~ | Excplicitly matches the end of the TOKENS list. |
|----------------------+--------------------------------------------------|
*** IF clauses
An ~IF~ clause lets the user check the values of a particular match
against a predicate. If the predicate is ~NIL~, the match fails.
An ~IF~ clause can be either a function designator or an arbitrary
S-EXPRESSION.
**** Example 1: Function Designator IF Clauses
#+begin_src
(<rule1>
:match (:item)
:if symbolp)
#+end_src
This would check that the token returned by matching against the
`(:item)` pattern is a symbol.
**** Example 2: Expression IF Clauses
#+begin_src
(<rule2>
:match (:seq (:= index-of) (:@ idx (:item)) (:@ str (:item)))
:if (and (integerp idx) (stringp str)))
#+end_src
This would match sequences like ~INDEX-OF 3 "Hello"~ and it would
ensure that ~3~, which gets bound to ~idx~ is an integer, and that
~"Hello"~ , which is bound to ~str~ , is a string.
E.g. ~INDEX-OF "Hello" 4~ would fail to match.
*** THEN clauses
A ~THEN~ clause lets users transform a match result. Just like an ~IF~
clause, it can be either a function designator or an arbitrary expression.
**** Example
#+begin_src
(<rule3>
:match (:seq
(:= :foo)
(:@ part1 <rule4>)
(:= :bar)
(:@ part2 <rule5>))
:if (and (good? part1) (also-good? part2))
:then (list part1 part2))
#+end_src
When ~<rule3>~ succeeds, it returns a list of two values.
*** The start rule
The very first rule in a ~deflanguage~ body is the ~START-RULE~. On a
successful parse of the ~TOKENS~, whatever the start rule results in is
what the macro being defined by ~deflanguage~ will expand in to.
|
