make 'then' optional in 'if' expressions?

In practice, I think I find then in if expressions significantly more annoying than I thought I would. And I’ve heard at least some complaints about it. The reason for requiring then was:

1. symmetry of the two branches of the if, and
2. to help resolve a grammatical ambiguity with the syntax of comprehensions.

I now no longer care about 1. I do still care about 2, but I must admit that we use comprehensions much less than I expected, and if expressions much more, so I guess I’m prepared to sacrifice some convenience in comprehensions in order to get a little convenience in if expressions. (It’s somewhat a tough call, however.)

The issue with comprehensions is the interpretation of the following code:

for (x in xs) if (fun(x)) x else 0

Does this mean:

for (x in xs) if (fun(x)) (x else 0)

Or:

for (x in xs) (if (fun(x)) x else 0)

The requirement for then neatly resolved that ambiguity in favor of the first interpretation. However, there was always the option of resolving it in the opposite direction, interpreting (if (fun(x)) x else 0 as an if expression instead of a clause of the comprehension.

The way to do that is to specify that the expression clause of a comprehension is not a full expression, but rather what the grammar calls a conditionalBranch. You can write almost any expression in a conditionalBranch except:

• anonymous functions
• then/else operators
• assignments
• switch expressions

Currently the branches of an if, switch, or let are conditionalBranches, but the expression clause of a comprehension is a full functionOrExpression.

So what this boils down to is, if I recall everything correctly:

• we could let you leave out the then in an if expression,
• but only if we required you to put parens around anonymous functions, the else operator, and switches in the expression clause of a comprehension.

Is there any support here for making a change like that?

[phlopsi]

Please, use the +1 or -1 reaction directly below the first post to express your opinion in a way, that’s easy to count. 😃

[CPColin]

I support leaving it as if-then-else.