First-class switch statement

[rachitnigam]

Calyx currently supports multi-armed if statements but often times, frontend code would benefit from a first class switch statement instead. For example, this pattern seems to come up a few times:

par {
  if x.out with cond0 { .. }
  if x.out with cond1 { .. }
  ...
}

Where cond0 and cond1 are really just doing the same comparison (think unrolled accesses generated from the Dahlia frontend). A better solution would be to have a switch-case style construct:

switch x.out {
  0 => { ... }
  1 => { ... }
}

if can remain surface syntax that is compiled to switch statements.

[sampsyo]

Seems pretty reasonable! But one thing I’m a little confused about: won’t we need some (moral equivalent of) the with clause? That is, the construct will need to know which group to activate to obtain x.out, right? (Even if it only activates it once and shares the result.)

[rachitnigam]

Sure, we can have a switch <port> with <group> construct when the <group> is a combinational group. I almost titled this discussion "Calyx 2" because we have now proposed a bunch of substantial changes to the language and the semantics that would be worth incorporating.