std.range.generate's range calls its argument one time too many

[dlangBugzillaToGithub]

snarwin+bugzilla (@pbackus) reported this on 2019-01-15T21:59:11Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=19587

CC List

• dkorpel
• remi.thebault

Description

Example:

---
int count = 0;

int counter()
{
    return count++;
}

void main()
{
    import std.algorithm;
    import std.range;
    import std.stdio;
    
    auto gen = generate!counter;
    gen.take(3).each!writeln;
    assert(count == 3); // fails, count == 4
}
---

This behavior leads to two related problems:

1) Ranges, in general, are expected to be lazy. Eagerly calling the generator function on construction and in popFront violates that expectation.

2) As a result, writing optimal code using ranges created with `generate` (that is, code which does no unnecessary work) requires special-case handling, since Phobos' algorithms (for example, `take`, above) "naively" assume that construction and popFront are cheap.

These problems are especially bothersome when the generator function is expensive to call (for example, if it accesses the network).

[dlangBugzillaToGithub]

dkorpel commented on 2021-05-06T22:53:27Z

Just got hit by this today, my `generate!(() => readln()).take(10).array` discarded the line after the 10 lines put into the array.

This is really annoying, but changing this would be a breaking change, so I'm afraid the fix would be either:

- a template parameter 'caching' with default value of `true`
- a new synonym symbol, similar to `approxEqual => isClose`, with the possibility to deprecate the old symbol (caching can still be achieved by doing `generate!(f).cache`). I'm not sure what that synonym would be, things that come to mind are `generateLazy`, `successiveCalls`, `repeatEvaluate`, but I'm not a big fan of them.

[dlangBugzillaToGithub]

remi.thebault commented on 2022-05-06T16:50:40Z

*** Issue 23094 has been marked as a duplicate of this issue. ***

[dlangBugzillaToGithub]

dlang-bot commented on 2022-05-06T17:37:06Z

@rtbo created dlang/phobos pull request #8453 "fix issue 19587" fixing this issue:

- fix issue 19587
  
  avoid that std.range.generate calls
  fun one time too many

https://github.com/dlang/phobos/pull/8453