[erlang-questions] Guards syntax for multiple values

[Jesper Louis Andersen]

On Wed, Mar 27, 2019 at 2:17 AM José Valim <jose.valim@REDACTED>
wrote:

> > Everybody seems to be chasing after how to express "X in [...]" as a
> guard
> without responding to the suggestion of copying SML/NJ's "or patterns"
>
> What are the restrictions for OR patterns?
> Can OR patterns be allowed anywhere in a pattern (i.e. arbitrarily nested)
> or are they allowed only at the root?
> Can we bind variables inside OR patterns?
>
>
I'll use Ocaml as the language for this, since it has the same pattern as
SML/NJ, and I happen to have Ocaml installed, but not SML/NJ.

In erlang we write:

case Exp of
  P1 -> E1;
  P2 -> E2;
  P3 -> E3
end

In Ocaml, we would write

match Exp With
| P1 -> E1
| P2 -> E2
| P3 -> E3

Note that we prefix each pattern by '|' rather than separate them with ';'
as in Erlang. The semantics are otherwise the same at least up to a very
high level of equivalence. Now suppose, we have a case where we want to
write

match Exp with
| P1 -> E
| P2 -> E
| P3 -> E3

That is, both patterns P1 and P2 runs the same function body E. In this
case, Ocaml allows us to avoid the repetition, the so-called OR-pattern:

match Exp with
| P1 | P2 -> E
| P3 -> E3

What are the rules? Any use of a variable 'x' in E must be bound in the
scope and must have the same type. That is, if P1 binds 'x : t' i.e., with
type t, then P2 must also bind 'x : t'. In principle, we could allow it if
the scope/context has 'x : t' and it isn't bound in P2, but this is
currently rejected by the language, probably on the grounds of being prone
to error.

The compiler has a relatively easy rewrite to make:

let body_E x = E in
  match Exp with
  | P1 x -> body_E x
  | P2 x -> body_E x
  ...

more or less (caveat: capture! body_E must be be passed parameters
appropriately if it uses them from the context. Further optimizations might
then lift them). Extract the common body into a local function and apply
that function. Further optimizations are often able to remove 'body_E' from
the generated code. Since we are in ML-land with Standard ML and Ocaml, the
Mlton compiler has a contification optimization which will fuse the body_E
continutation appropriately. This can then uncover further optimizations
down the road.
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