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go/parser: confusing first error message #3106

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griesemer opened this issue Feb 23, 2012 · 3 comments
Closed

go/parser: confusing first error message #3106

griesemer opened this issue Feb 23, 2012 · 3 comments
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@griesemer
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@griesemer griesemer commented Feb 23, 2012

What steps will reproduce the problem?
- use gofmt (for instance) to parse the program below

What is the expected output? What do you see instead?
-  the first error message is confusing; the actual error is the 2nd error
- should try to handle cases like this better

Test case:

package main

func f() {
    var m Mutex
    c := MakeCond(&m)
    percent := 0
    const step = 10
    for i := 0; i < 5; i++ {
        go func() { // #### FIRST ERROR REPORTED HERE
            for {
                // Emulates some useful work.
                time.Sleep(1e8)
                m.Lock()
                defer // #### ACTUAL PROBLEM IS HERE
                if percent == 100 {
                    m.Unlock()
                    break
                }
                percent++
                if percent % step == 0 {
                    //c.Signal()
                }
                m.Unlock()
            }
        }()
    }
    for {
        m.Lock()
        if percent == 0 || percent % step != 0 {
            c.Wait()
        }
        fmt.Print(",")
        if percent == 100 {
            m.Unlock()
            break
        }
        m.Unlock()
    }
}

gofmt -l example_test.go 
example_test.go:9:6: expected function/method call
example_test.go:15:5: expected function/method call
example_test.go:25:4: expected ';', found '('
example_test.go:27:2: expected declaration, found 'for'
example_test.go:28:3: expected declaration, found 'IDENT' m
example_test.go:29:3: expected declaration, found 'if'
example_test.go:30:4: expected declaration, found 'IDENT' c
example_test.go:31:3: expected declaration, found '}'
example_test.go:32:3: expected declaration, found 'IDENT' fmt
example_test.go:33:3: expected declaration, found 'if'
example_test.go:34:4: expected declaration, found 'IDENT' m
example_test.go:35:4: expected declaration, found 'break'
example_test.go:36:3: expected declaration, found '}'
example_test.go:37:3: expected declaration, found 'IDENT' m
example_test.go:38:2: expected declaration, found '}'
example_test.go:39:1: expected declaration, found '}'
@rsc
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@rsc rsc commented Feb 23, 2012

Comment 1:

I'm not sure this is an ordering problem so much as
just getting too confused by a lone defer keyword.
If the parser realized that it should give up on the defer
and start a new statement at the if token, then I think you'd
get just the one error.
@griesemer
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@griesemer griesemer commented Feb 23, 2012

Comment 2:

Agreed. I was not implying that this is an ordering problem. What happens is that the
parse of the function call for the go statement returns a function literal instead of a
call, which then causes the spurious first error message (first only because the
messages are sorted by position). The parser should either synchronize better when
encountering the flawed defer; and/or return a BadExpr for the function call which could
be used as a signal to not issue another error.
@griesemer
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@griesemer griesemer commented Mar 7, 2012

Comment 3:

This issue was closed by revision c8981c7.

Status changed to Fixed.

@griesemer griesemer added fixed labels Mar 7, 2012
@griesemer griesemer self-assigned this Mar 7, 2012
@golang golang locked and limited conversation to collaborators Jun 24, 2016
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