请求为自定义SWL语言提供合法的ANTLR if语句语法

Hey there! Looking at your existing SWL grammar, I notice you've got ifcond listed as a valid statement type but haven't defined its rule yet. Let's fix that while keeping consistent with SWL's natural-language style (like let ... be, add ... to) that you've already established.

Updated Grammar with If Statement Rules #

Here's the modified grammar including the complete ifcond definition:

grammar swl;

program : 'begin' statement+ 'end';
statement : assign | add | sub | mul | div | print | whilecond | ifcond ;
condition : expr ;

// Existing expression rule remains unchanged
expr : '(' expr ')' 
     | expr 'and' expr 
     | expr 'or' expr 
     | 'not' expr 
     | expr '=' expr 
     | expr '>=' expr 
     | expr '<=' expr 
     | expr '>' expr 
     | expr '<' expr 
     | ID 
     | NUMBER;

assign : 'let' ID 'be' (NUMBER | ID) ;
print : 'print' (NUMBER | ID) ;
add : 'add' (NUMBER | ID) 'to' ID ;
// Assume you'll define sub, mul, div similarly

// New if statement rule
ifcond : 'if' condition 'then' statement+ ('else' statement+)? 'fi';

Key Design Choices Explained #

• Natural Language Alignment: Uses if ... then ... else ... fi to match SWL's conversational keyword pattern (mirroring how begin/end wraps the whole program).
• Optional Else Branch: The ('else' statement+)? makes the else clause optional, so you can write both simple ifs and full if-else logic.
• Multi-Statement Blocks: Both the then and else sections accept statement+, meaning you can put multiple commands inside each condition block (just like the main program's begin/end).
• Explicit Closure: The fi keyword closes the if statement, avoiding ambiguity with nested conditions and keeping the grammar unambiguous.

Example Valid SWL If Statements #

To help you visualize how this works, here are some valid uses of the new if syntax:

• Simple condition with single statement:

  if score >= 90 then print "Excellent" fi
  

• If-else with multiple statements:

  if balance < 0 then
      print "Overdrawn"
      let fee be 20
      add fee to balance
  else
      print "Balance is positive"
  fi
  

• Nested if conditions:

  if temp > 30 then
      if humidity > 70 then
          print "Hot and humid"
      else
          print "Hot but dry"
      fi
  fi
  

内容的提问来源于stack exchange，提问作者Emma Rossignoli