iter: iter.Seq returns incorrect values for subsequent next calls

[j178]

Go version

go version devel go1.22-46ea4ab Sat Dec 9 21:48:06 2023 +0000 darwin/amd64

What operating system and processor architecture are you using (go env)?

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/Users/jo/Library/Caches/go-build'
GOENV='/Users/jo/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT='rangefunc'
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/jo/go/pkg/mod'
GONOPROXY=''
GOOS='darwin'
GOPATH='/Users/jo/go'
GOPRIVATE=''
GOROOT='/Users/jo/sdk/gotip'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/jo/sdk/gotip/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='devel go1.22-46ea4ab Sat Dec 9 21:48:06 2023 +0000'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='0'
GOMOD='/Users/jo/code/it/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/sw/0cn9x8611_xdn6c20pxd96m40000ks/T/go-build14739506649094477967=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

// GOEXPERIMENT=rangefunc

package main

import "iter"

func Range(end int) iter.Seq[int] {
	i := 0
	return func(yield func(int) bool) {
		for ; i < end; i++ {
			if !yield(i) {
				break
			}
		}
	}
}

func Next(seq iter.Seq[int]) (int, bool) {
	for v := range seq {
		return v, true
	}
	return 0, false
}

func main() {
	r := Range(2)

	v, ok := Next(r)
	println(v, ok)
	v, ok = Next(r)
	println(v, ok)
	v, ok = Next(r)
	println(v, ok)
}

What did you expect to see?

0 true
1 true
0 false

What did you see instead?

0 true
0 true
0 true

[gophun]

You're restarting iteration with each range. If you want to turn the push-based iter.Seq into a pull-based iterator use the iter.Pull function on it.

[prattmic]

cc @rsc

[j178]

I realized that the yield function always returns false, the loop never progresses beyond the first iteration, resulting in an infinite sequence of 0. The corrected implementaion of Range:

func Range(end int) iter.Seq[int] {
	i := 0
	return func(yield func(int) bool) {
		for ; i < end; i++ {
			if !yield(i) {
				i++ // proceed here
				break
			}
		}
	}
}