Original bug ID: 6357
Reporter: @johnwhitington
Status: closed (set by @mshinwell on 2016-12-08T12:25:32Z)
Resolution: won't fix
Priority: normal
Severity: feature
Category: ~DO NOT USE (was: OCaml general)
Related to: #6685
Monitored by: @ygrek

Bug description

When doing, for example, input with imperative constructs, there seem to be three ways to test the input for end of data as part of the conditional, whilst also using its value in the loop:

let fin = ref false
while !fin do
match input_byte () with
x when x = eod -> fin := true
| x -> ...x...
done

(pollutes scope with name 'fin', even though not relevant outside loop body)

exception Finished

try
while true do
match input_byte () with
x when x = eod -> raise Finished
| x -> ...x...
done
with
Finished -> ......

(needs an exception - same one each time, or define a new one each time?)

let x = ref 0 in
while x := input_byte (); !x <> eod do
...!x...
done

(pollutes scope with name 'x', even though not relevant outside loop body)

It would be nice to be able to bind part of the expression to a name, like this:

while input_byte () as x <> eod do
...x...
done

Now x is only available in the loop body, which is neater.