Can you use go run for scripting? I wrote a small program to find out.

package main

import (
    "fmt"
    "os"
    "os/exec"
    "time"
)

func main() {
    start := time.Now()
    gogo := fmt.Sprintf(`package main
    import "fmt"
    func main() {
        fmt.Printf("the time was %s\n")
    }`, start)
    os.WriteFile("gogo.go", []byte(gogo), 0666)
    cmd := exec.Command("go", "run", "gogo.go")
    cmd.Stdout = os.Stdout
    cmd.Run()
    end := time.Now()
    fmt.Printf("that took %s\n", end.Sub(start))
}

$ go run gorun.go
the time was 2024-01-05 12:04:01.042488 -0500 EST m=+0.000077959
that took 329.81325ms

On my somewhat older m3 running openbsd, it takes about 330ms. On a less old chromebook, it takes about 100ms. Not bad. On an M1 macbook, it varies wildly between 150ms and 300ms.

So I think not fast enough for interactive use. Probably fast enough for a continuously changing cronjob, though I wonder why you wouldn’t change the inputs. But I guess if you’re a big believer in configuration through compilation, and also have constantly changing requirements, it’s fast enough to get by.

For comparison, yaegi is still 10x or more quicker at putting something on the screen.

go build

I almost exclusively run go build. Actually, I run make and that runs go build, because that’s how my fingers work. So it wasn’t until recently that I noticed that go run is as fast as it is. I was used to a very observable one second per build.

Turns out go build will run hg stat for every build, which go run skips, and that was the slowest part of the process. Easily remedied by switching to chg.

Go 1.21 added experimental support for fixing the loop capture bug. For reasons it’s never really bothered me, but the other loop bug does bite me some times. This is the one where the loop values are values.

I want references but go doesn’t have references.

values

The problem is easy to see when you’re looking for it, and hard to see when you’re glossing over the code.

func resetScores(players []Player) {
    for _, player := range players {
        player.score = 0
    }
}

Before and after “the” loop fix, this will dutifully set the player’s score to zero. But which player? An anonymous player that only exists in the loop. The array will remain unchanged. Ugh.

hand fix

Fixing this requires slightly rewriting the loop.

func resetScores(players []Player) {
    for i := range players {
        players[i].score = 0
    }
}

This looks kinda weird I think, and it’s not my first instinct to write it this way. I really just want a way to say hi, I’m iterating over the elements of this array, give me a reference to them.

mechanical fix

So let’s add references to go.

func resetScores(players []Player) {
    for _, &player := range players {
        player.score = 0
    }
}

This can be translated by powerful state of the art tools into the go you’d have to write otherwise.

func resetScores(players []Player) {
    for  _i, _ := range players {
        player := &players[_i]
        player.score = 0
    }
}

oh well

What’s annoying is the above broken form works if the array contains pointers. A lot of my code uses pointers, so I get used to writing the first form, and then it breaks when I do something different. Or I decide to optimize the code and reduce allocations by switching from []*Player to []Player without a careful audit of the impacted loops. Alas, I probably wouldn’t think to use the reference form even if available until it’s too late.

The common mistakes wiki lists the capture bug twice, but doesn’t mention this bug.

I like using pledge and unveil in my web apps. Especially unveil offers a nice degree of protection against common web app problems, like the dreaded double dot traversal. For go, I use a simple wrapper which gets pasted into each project.

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