OK, I'll make you a deal: I'll answer your question about let/bind,
under the condition that we not divert the discussion on that right now
-- there'll be a proper writeup soon. The answer here is entirely for
context.
If you don't agree, stop reading now :)
On 2/15/2022 5:58 PM, Remi Forax wrote:
- There are future constructs that may take patterns, and may (or
may not) want to express guard-like behavior, such as `let`
statements (e.g., let .. when .. else.) Expressing guards here
with && is even less evocative of "guard condition" than it is
with switches.
It's not clear to me how to use "let when else". Is it more like a ?:
in C than the let in in Caml ?
The simplest form of `let` is a statement that takes a total pattern:
let Point(var x, var y) = aPoint;
and introduces bindings x and y into the remainder of the block. When
applicable, this is better than a conditional context because (a) you
get type checking for totality, and (b) you don't indent the rest of
your method inside a test that you know will always succeed.
If the pattern is total but has some remainder, the construct must throw
on the remainder, to preserve the invariant that when a `let` statement
completes normally, all bindings are DA.
What if I want to use a partial pattern, and then customize either the
throwing part or provide default values? I can provide an else clause:
Object o = ...
let String s = o
else throw new NotStringException();
or
Object o = ...
let String s = o
else { s = "no string"; }
These are two ways to preserve the "DA on normal completion" invariant;
either by not completing normally, or by ensuring the bindings are DA.
Now, we are in a situation where we are with switch: patterns do not
express all possible conditions. Which is why we introduced guards to
switches. And we can use the same trick here:
Object o = ...
let String s = o
when (!s.isEmpty())
else { s = "no string"; }
If we tried to use && here, it would look like
Object o = ...
let String s = o && (!s.isEmpty())
else { s = "no string"; }
which has the same problem as `case false && false`.
Reminder: THIS EXPLANATION WAS PROVIDED SOLELY TO CLARIFY THE "FUTURE
CONSTRUCT" COMMENT IN THE && DISCUSSION.
