S-expressions: Difference between revisions

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 However, note that the <code>@</code> character has a special meaning: <code>@#</code> turns into <code>(sys:var #)</code>. TXR's printer right now does not convert this back to <code>@</code> notation upon printing. (The purpose of this notation is to support Lisp code that requires meta-variables: variables distinguished from variables. For instance, logic pattern matching or unification code. Instead of hacks like name-based conventions (for instance <code>x?</code> is a meta-variable, <code>x</code> is ordinary), why not build it into the language: <code>@x</code> is a meta-var, identifiable by special abstract syntax, and <code>x</code> is just an atom, a symbol. There is also <code>@(foo ...)</code> which expands into <code>(sys:expr foo ...)</code>, doing a similar thing for expressions.
-(TODO: A solution will appear here soon which avoids "cheating" in this way with the built-in parser: a from-scratch S-exp parser which treats <code>!@#</code> as just a symbol. We can beat other languages with our built-in S-exp parser tied behind our back.)
+The following solution avoids "cheating" in this way with the built-in parser; it implements a from-scratch S-exp parser which treats <code>!@#</code> as just a symbol.
+The grammar is as follows:
+<pre>
+expr := ws? atom
+     |  ws? ( ws? expr* ws? )
+atom := float | int | sym | str
+float := sign? digit+ . digit* exponent?
+      |  sign? digit* . digit+ exponent?
+      |  sign? digit+ exponent
+int := sign? digit+
+str := " (\" | anychar )* "
+sym := sym-char +
+sym-char := /* non-whitespace, but not ( and not ) */</pre>
+Code:
+<lang txr>@(define float (f))@\
+  @(local (tok))@\
+  @(cases)@\
+    @{tok /[+\-]?\d+\.\d*([Ee][+\-]?\d+)?/}@\
+  @(or)@\
+    @{tok /[+\-]?\d*\.\d+([Ee][+\-]?\d+)?/}@\
+  @(or)@\
+    @{tok /[+\-]?\d+[Ee][+\-]?\d+/}@\
+  @(end)@\
+  @(bind f @(flo-str tok))@\
+@(end)
+@(define int (i))@\
+  @(local (tok))@\
+  @{tok /[+\-]?\d+/}@\
+  @(bind i @(int-str tok))@\
+@(end)
+@(define sym (s))@\
+  @(local (tok))@\
+  @{tok /[^\s()]+/}@\
+  @(bind s @(intern tok))@\
+@(end)
+@(define str (s))@\
+  @(local (tok))@\
+  @{tok /"(\\"|[^"])*"/}@\
+  @(bind s @[tok 1..-1])@\
+@(end)
+@(define atom (a))@\
+  @(cases)@\
+    @(float a)@(or)@(int a)@(or)@(str a)@(or)@(sym a)@\
+  @(end)@\
+@(end)
+@(define expr (e))@\
+  @(cases)@\
+    @/\s*/@(atom e)@\
+  @(or)@\
+    @/\s*\(\s*/@(coll :vars (e))@(expr e)@/\s*/@(until))@(end))@\
+  @(end)@\
+@(end)
+@(freeform)
+@(expr e)@junk
+@(output)
+expr: @(format nil "~s" e)
+junk: @junk
+@(end)</lang>
+Run:
+<pre>$ txr s-expressions.txr -
+()
+expr: nil
+junk:
+$ txr s-expressions.txr -
+e3
+expr: 3000.0
+junk:
+$ txr s-expressions.txr -
++3
+expr: 3
+junk:
+$ txr s-expressions.txr -
+abc*
+expr: abc*
+junk:
+$ txr s-expressions.txr -
+abc*)
+expr: abc*
+junk: )
+$ txr s-expressions.txr -
+((data "quoted data" 123 4.5)
+ (data (!@# (4.5) "(more" "data)")))
+expr: ((data "quoted data" 123 4.5) (data (!@# (4.5) "(more" "data)")))
+junk:
+</pre>
 {{omit from|Brlcad}}