Add grammar for "if (test) then {expr}" with no else

[michaelhkay]

As discussed in issue #234. In reviewing this PR, I suggest we consider it together with the existing proposals for ternary conditionals (x ?? y !! z) and the "otherwise" operator.

[ChristianGruen]

If curly braces are used, I would strongly vote for allowing the else branch. Due to the braces, we now have an unambiguous assignment of conditions and branches. Examples:

Expression	Equivalent expression
if($a) { if($b) { $c } }	if($a) then if ($b) then $c else () else ()
if($a) { if($b) { $c } else { $d } }	if($a) then if ($b) then $c else $d else ()
if($a) { if($b) { $c } } else { $e }	if($a) then if ($b) then $c else () else $e
if($a) { if($b) { $c } else { $d } } else { $e }	if($a) then if ($b) then $c else $d else $e

[ChristianGruen]

In addition, we should allow else if:

if($a) { $b } else if($c) { $d } else { $e }

[ChristianGruen]

Thanks, @michaelhkay, for adding else. I have revised my first comment to make it better accessible.

In my second comment, I proposed to also support else if chains:

if($a) { $b }
else if($c) { $d }
else if($e) { $f }

The grammar rules for else if could look as follows:

IfExpr       ::= "if" "(" Expr ")" (("then" ExprSingle "else" ExprSingle) | BracedIfExpr)
BracedIfExpr ::= EnclosedExpr ("else" (EnclosedExpr | ("if" "(" Expr ")" BracedIfExpr)))?

Without the extended grammar, we would need to use additional curly braces:

if($a) {
  $b
} else {
  if($c) {
    $d
  } else {
    if($e) {
      $f
    }
  }
}

[liamquin]

i like elsif or elif but i think there's probably no ambiguity (neither for humans nor for the parser) with else if, so sounds OK here.

[michaelhkay]

Of all the languages I've used, I much preferred Algol68's IF expr (THEN expr)? (ELSE expr)? FI. Sigh.

[michaelhkay]

As regards the grammar, we tend to prefer iteration over recursion. So perhaps

IfExpr       ::= "if" "(" Expr ")" (UnbracedActions | BracedActions)
UnbracedActions ::= "then" ExprSingle "else" ExprSingle 
BracedActions ::= EnclosedExpr ("else" "if "(" Expr ")" EnclosedExpr)*  ("else" EnclosedExpr)?

[ChristianGruen]

+1 for the iterative variant of the grammar.

The year 1968 is something I wasn't blessed to experience personally, but when I read FI, I get reminded of good old machine code (beq/bne).

[ndw]

Accepted at meeting 018