4 ms·

You could make `else if` a special sequence of tokens that works like `elif`.

• cxr 5y ago
  Why are people upvoting this as if it wasn't just explained why that's not true? That's only possible when you have (mandatory) block delimiters. That was the entire basis of the comment—that `elif` is used in "languages that don't have an explicit block statement with its own delimiters".
  • xigoi 5y ago
    How is it not possible? Here's a simple grammar: if_stmt <- "if" expr INDENT stmts DEDENT ("else" "if" expr INDENT stmts DEDENT)* ("else" INDENT stmts DEDENT)? It does require one-token lookahead, but it's unambiguous.
    • deleted 5y ago
      [deleted]
    • cxr 5y ago
      Are INDENT and DEDENT here describing whitespace sensitivity in the vein of Python, which "uses whitespace to delimit control flow blocks"[1] (elsewhere[2]: "Python uses whitespace indentation, rather than curly brackets or keywords, to delimit blocks")? We're specifically talking about the subset of languages where these blocks _don't_ have their own delimiters[3]. 1. https://en.wikipedia.org/w/index.php?title=Python_syntax_and https://en.wikipedia.org/w/index.php?title=Python_syntax_and... 2. https://en.wikipedia.org/w/index.php?title=Python_(programmi https://en.wikipedia.org/w/index.php?title=Python_(programmi... 3. There is a reading of Bob's original comment such that it's suggesting that Python and end-if languages are among languages without them (or something—I can't make total sense of that part of the comment; it gets kind of garden path-y), but they're not.
      • xigoi 5y ago
        It doesn't matter how indentation is parsed. What I wanted to say that you can simply replace "elif" with "else" "if" in the grammar.
    • munificent 5y ago
      You're right, you could do this in Python because it uses indentation for block delimiting. That means this is unambiguous: if c1 then: s1 else if c2: s2 s3 Without significant indentation, it's not possible to tell if "s3" is indented to be part of the the second if's then branch. In other words, "else if" and "else INDENT if" are unambiguously different in Python. That's not true in other languages that use "elif"/"elsif"/"elseif" where only newlines are significant and not indentation.
• munificent 5y ago
  You can, yes. However, doing that requires making the newline between the `else` and `if` significant: if c1 then s1 else if c2 then s2 else s3 end end // <-- Two "end"s Versus: if c1 then s1 else if c2 then s2 end // <-- Only one "end" If you don't make the newline there significant, then every if statement at the beginning of an else clause would be interpreted as chained: if c1 then s1 else if c2 then s2 end s3 end Here, the parser will incorrectly treat the first "end" as closing the outer "if" when it's intended to close the inner one. It will then think "s3" is outside of the "else" clause and then consider the second "end" to be a parse error. In principle, this isn't actually ambiguous and if the parser had unbounded lookahead it could probably find the second "end" and rollback and figure out what's going on. In practice, languages with unbounded lookahead are hard for humans to parse too. Having an explicit "elif" token avoids all of this weirdness.