Add 'delay' parser to peg

extra/peg/peg-docs.factor

@@ -138,3 +138,13 @@ HELP: hide
     "Returns a parser that succeeds if the original parser succeeds, but does not " 
     "put any result in the AST. Useful for ignoring 'syntax' in the AST." }
 { $example "\"[\" token hide number \"]\" token hide 3array seq" } ;
+
+HELP: delay
+{ $values 
+  { "quot" "a quotation with stack effect ( -- parser )" } 
+  { "parser" "a parser" } 
+}
+{ $description 
+    "Delays the construction of a parser until it is actually required to parse. This " 
+    "allows for calling a parser that results in a recursive call to itself. The quotation "
+    "should return the constructed parser." } ;

extra/peg/peg.factor

@@ -168,6 +168,11 @@ TUPLE: sp-parser p1 ;
 M: sp-parser parse ( state parser -- result )
   [ left-trim-slice ] dip sp-parser-p1 parse ;
 
+TUPLE: delay-parser quot ;
+
+M: delay-parser parse ( state parser -- result )
+  delay-parser-quot call parse ;
+
 PRIVATE>
 
 : token ( string -- parser )
@@ -208,3 +213,6 @@ PRIVATE>
 
 : hide ( parser -- parser )
   [ drop ignore ] action ;
+
+: delay ( parser -- parser )
+  delay-parser construct-boa init-parser ;