Search code examples
c#parsinggrammarebnfirony

How can I define an "else" block?

Here's what I'm trying:

foreach_in.Rule = ToTerm("foreach") + "(" + VARIABLE + "in" + list_obj + ")";
foreach_as.Rule = ToTerm("foreach") + "(" + list_obj + "as" + VARIABLE + ")";
for_loop.Rule = ToTerm("for") + "(" + simple_assignment + ";" + comparison + ";" + assignment + ")";
if_condition.Rule = ToTerm("if") + "(" + comparison + ")";
if_else.Rule = if_condition + block + "else"; // <-- PROBLEM
preset_directive.Rule = foreach_in | foreach_as | for_loop | if_else | if_condition;
directive.Rule = preset_directive | custom_directive;
directive_blk.Rule = directive + block;

But I get a shift-reduce conflict. I'm not quite sure why... shouldn't it greedily grab the "else" if it can? Not quite sure how else to define an else block such that it can only be followed by an "if" block.

I think an if_else block node with an if node and an else node would be optimal because then I don't have to go back and check the previous sibling when I try traverse the AST.

If you need to see more of the grammar...just let me know. A "block" is basically defined as { blah } (a bunch of statements between {}).

Trying it as an optional block:

custom_directive_kw.Rule = ToTerm("custom_directive1") | "custom_directive2";
custom_directive.Rule = custom_directive_kw + free_args_opt;
foreach_in.Rule = ToTerm("foreach") + "(" + variable + "in" + list_obj + ")" + block;
foreach_as.Rule = ToTerm("foreach") + "(" + list_obj + "as" + variable + ")" + block;
for_loop.Rule = ToTerm("for") + "(" + simple_assignment + ";" + comparison + ";" + assignment + ")" + block;
if_condition.Rule = ToTerm("if") + "(" + comparison + ")" + block + else_blk_opt;
else_blk.Rule = "else" + block;
else_blk_opt.Rule = else_blk | Empty;
preset_directive.Rule = foreach_in | foreach_as | for_loop | if_condition;
directive.Rule = preset_directive | custom_directive;
directive_blk.Rule = directive;

Doesn't like that either. Still throws the warning.

Solution

  • Nevermind... Irony has this magical PreferShiftHere() function that does the trick.

    foreach_in.Rule = ToTerm("foreach") + "(" + variable + "in" + list_obj + ")" + block;
foreach_as.Rule = ToTerm("foreach") + "(" + list_obj + "as" + variable + ")" + block;
for_loop.Rule = ToTerm("for") + "(" + simple_assignment + ";" + comparison + ";" + assignment + ")" + block;
if_condition.Rule = ToTerm("if") + "(" + comparison + ")" + block;
if_else.Rule = if_condition + PreferShiftHere() + "else" + block;
preset_directive.Rule = foreach_in | foreach_as | for_loop | if_else | if_condition;
directive_blk.Rule = preset_directive | custom_directive;