evaluator: it looks like the result of executing functions is not reused

[bozaro]

What version of CUE are you using (cue version)?

$ cue version

cue version v0.0.0-20230816083946-7d801ba406ea

go version go1.20.7
      -buildmode exe
       -compiler gc
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
             vcs git
    vcs.revision 7d801ba406ea82923c19e66ce6aa419fdb62b2e8
        vcs.time 2023-08-16T08:39:46Z
    vcs.modified true

Does this issue reproduce with the latest stable release?

Yes

What did you do?

I evaluate cue file with breakpoint on line: https://github.com/cue-lang/cue/blob/v0.6.0/pkg/strings/pkg.go#L300

import "strings"

let foo = strings.HasPrefix("foo", "bar")
a: foo
b: foo
c: foo
d: foo
e: foo

What did you expect to see?

I expect only one interrupt at breakpoint (function strings.HasPrefix executed one time).

What did you see instead?

I got 6 interrupts at breakpoints (function strings.HasPrefix executed 6 times).

[mvdan]

Perhaps related to #2243, where "chained" calls to builtins would result in increasingly slower evaluation.

[bozaro]

I checked behaviour before and after changeset https://review.gerrithub.io/c/cue-lang/cue/+/549247 from #2243 and got exactly same behaviour.

[bozaro]

Interestingly, in this example, strings.ToLower is executed exactly once:

import "strings"

let bar = strings.ToLower("BAR")
a: strings.HasPrefix("foo1", bar)
b: strings.HasPrefix("foo2", bar)
c: strings.HasPrefix("foo3", bar)
d: strings.HasPrefix("foo4", bar)
e: strings.HasPrefix("foo5", bar)

But in this example strings.ToLower is executed six times:

import "strings"

let bar = strings.ToLower("BAR")
a: bar
b: bar
c: bar
d: bar
e: bar

[bozaro]

I spent several days debugging this issue and wrote the ugly workaround:

• Ugly workaround for #2566 issue #2589