[erlang-questions] -protocol declartion idea, worth an EEP?
Richard A. O'Keefe
Tue Jun 6 00:42:49 CEST 2017
> On 6/06/2017, at 5:35 AM, Michael Truog <mjtruog@REDACTED> wrote:
> I believe it is more important to be able to mark functions impure or pure
Actually that was something I was working on with SERC just before
funs were introduced into Erlang and completely derailed my ideas.
One issue is that 'pure' or 'impure' is far too coarse a classification.
For example, we might want to distinguish
- terminates in bounded time
- terminates in finite time
- not known to terminate
- process dictionary
- no dependency on process dictionary
- reads specified parts of process dictionary
- writes (and maybe reads) specified parts of process dictionary
- does no I/O
- opens and closes
- inter-process communication
- does it create processes or not
- does it send messages or not
- does it receive messages (which?) or not
And that isn't meant to be a complete list.
Another issue is higher-order functions.
map(F, [X|Xs]) -> [F(X) | map(F, Xs)];
map(_, ) -> .
is itself pure, but if F has any effects, map(F, ...) may
(but need not) have those effects too. So we need a type
map/2 :: (A -> B effect E, list(A)) -> list(B) effect E+finite.
Since then, Erlang has gone through at least three different
type systems (the Wadler one, the Armstrong one, and the one
it now has), and I never did get around to trying to adapt a
proper "effect" system to the Dialyzer approach.
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