- The (optional) abstract syntax of a language
may be specified by a simpler grammar than that for the concrete syntax.
<Exp> + <Exp>
| <Exp> - <Exp>
| <Exp> * <Exp>
| <Exp> / <Exp>
| - <Exp>
A grammar for the abstract syntax of simple arithmetic expressions
- Compared to the
the abstract syntax is ambiguous and is unsuitable
for direct use in building a parser.
However it is simpler than the concrete syntax and
is very suitable to build
the data-structure for the parse-tree returned by a parser, e.g.,
- datatype Exp = binexp of Exp * Bopr * Exp
| unexp of Uopr * Exp
| varid of Ide | ...
- For example, given the input x+y*z either of the
following trees is possible according to the abstract syntax above
- Parse trees for x + y * z
- but only the left tree is correct according to
the conventional concrete syntax.
- However the right tree would be the correct one given the input
Note that there are no parentheses in the
abstract syntax above nor in the parse trees.
Parentheses are only needed to direct the parser to a certain parse and
are not required in a parse tree which shows the binding of
operators to operands by its very structure.
window on the wide world:|
|| "can be replaced by"
| -> |
| | || "or by" |
| => || derivation |
|| left most
|| right most