Cst
Cst is a concrete syntax tree, which means it unambiguously defines the program in a form very close to the syntax used by programmers or any other language users.
In the case study we model Ast by Rascal Concrete Syntax Definition, see http://tutor.rascal-mpl.org/Rascal/Expressions/ConcreteSyntax/ConcreteSyntax.html This specification utilises many features, including:
- layout which is automatically "ignored" during pattern matching
- explicit starting nonterminal which takes care of leading & trailing layout
- named subexpressions that make writing mappers easier
- explicit left associativity and production precedence to encode priorities
- follow restrictions for greedy matching of preterminals
- deep matching of ambiguities as a validation technique
module types::Cst
import ParseTree;
layout L = WS*;
lexical WS = [\ \t\n\r];
start syntax Cst = CstLHS lhs "=" CstRHS rhs ";";
syntax CstLHS = CstName f CstNameArgs args;
syntax CstRHS = CstExpr rhs;
syntax CstNameArgs = CstName+;
syntax CstExpr
= CstAtom a
> left CstExpr l "*" CstExpr r
> left CstExpr l "+" CstExpr r
;
syntax CstAtom
= CstName name
| CstNumber number
;
lexical CstName = [a-z]+ !>> [a-z];
lexical CstNumber = [0-9]+ !>> [0-9];
Cst example = parse(#start[Cst],"f arg = arg +1;").top;
Cst tricky = parse(#start[Cst],"f arg = 1+2*2+1;").top;
public bool validate(Cst p) = /amb(_) !:= p;
test bool vCst1() = validate(example);
test bool vCst2() = validate(tricky);