Name Recipes/Visualization/ParseTree

Synopsis Visualize a parse tree.

Description A parse tree is a (usually large) internal representation of a parsed text. In the rare situation that it is necessary to read or inspect a parse tree, a visualization can be useful.

Examples We embark on visualizing parse trees for the language Exp:

rascal>import demo::lang::Exp::Concrete::WithLayout::Syntax;
ok
rascal>import ParseTree;
ok
rascal>parse(#Exp, "1+2*3");
sort("Exp"): `1+2*3`
Tree: appl(prod(sort("Exp"),[sort("Exp"),layouts("Whitespace"),lit("+"),layouts("Whitespace"),sort("Exp")],{assoc(left())}),[appl(prod(sort("Exp"),[lex("IntegerLiteral")],{}),[appl(prod(lex("IntegerLiteral"),[iter(\char-class([range(48,57)]))],{}),[appl(regular(iter(\char-class([range(48,57)]))),[char(49)])[@loc=|file://-|(0,1,<1,0>,<1,1>)]])[@loc=|file://-|(0,1,<1,0>,<1,1>)]])[@loc=|file://-|(0,1,<1,0>,<1,1>)],appl(prod(layouts("Whitespace"),[\iter-star(\char-class([range(9,10),range(13,13),range(32,32)]))],{}),[appl(regular(\iter-star(\char-class([range(9,10),range(13,13),range(32,32)]))),[])[@loc=|file://-|(1,0,<1,1>,<1,1>)]])[@loc=|file://-|(1,0,<1,1>,<1,1>)],appl(prod(lit("+"),[\char-class([range(43,43)])],{}),[char(43)]),appl(prod(layouts("Whitespace"),[\iter-star(\char-class([range(9,10),range(13,13),range(32,32)]))],{}),[appl(regular(\iter-star(\char-class([range(9,10),range(13,13),range(32,32)]))),[])[@loc=|file://-|(2,0,<1,2>,<1,2>)]])[@loc=|file://-|(2,0,<1,2>,<1,2>)],appl(prod(sort("Exp"),[sort("Exp"),layouts("Whitespace"),lit("*"),layouts("Whitespace"),sort("Exp")],{assoc(left())}),[appl(prod(sort("Exp"),[lex("IntegerLiteral")],{}),[appl(prod(lex("IntegerLiteral"),[iter(\char-class([range(48,57)]))],{}),[appl(regular(iter(\char-class([range(48,57)]))),[char(50)])[@loc=|file://-|(2,1,<1,2>,<1,3>)]])[@loc=|file://-|(2,1,<1,2>,<1,3>)]])[@loc=|file://-|(2,1,<1,2>,<1,3>)],appl(prod(layouts("Whitespace"),[\iter-star(\char-class([range(9,10),range(13,13),range(32,32)]))],{}),[appl(regular(\iter-star(\char-class([range(9,10),range(13,13),range(32,32)]))),[])[@loc=|file://-|(3,0,<1,3>,<1,3>)]])[@loc=|file://-|(3,0,<1,3>,<1,3>)],appl(prod(lit("*"),[\char-class([range(42,42)])],{}),[char(42)]),appl(prod(layouts("Whitespace"),[\iter-star(\char-class([range(9,10),range(13,13),range(32,32)]))],{}),[appl(regular(\iter-star(\char-class([range(9,10),range(13,13),range(32,32)]))),[])[@loc=|file://-|(4,0,<1,4>,<1,4>)]])[@loc=|file://-|(4,0,<1,4>,<1,4>)],appl(prod(sort("Exp"),[lex("IntegerLiteral")],{}),[appl(prod(lex("IntegerLiteral"),[iter(\char-class([range(48,57)]))],{}),[appl(regular(iter(\char-class([range(48,57)]))),[char(51)])[@loc=...

As can be seen, even for such a trivial example, the details in the parse tree representation become sizeable.

We can visualize it as follows:

import demo::lang::Exp::Concrete::WithLayout::Syntax;
import ParseTree;
import vis::Figure;
import vis::ParseTree;
import vis::Render;
render(visParsetree(parse(#Exp, "1+2*3")));

With as result:

The figure is interactive (not available here):

• Rectangles with blue text are terminal symbols.
• Little circles represent non-terminals: hovering over them shows the corresponding grammar rule.
• Little grey rectangles represent layout: hovering over them also shows the corresponding lexical rule.

Benefits

• A dense, structured, representation of a parse tree that provides extra information via interaction.

Pitfalls

• This visualization does not scale to huge trees.

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