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fmt: mismatch between explicit argument index documentation and behaviour #9632

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kortschak opened this issue Jan 19, 2015 · 6 comments
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@kortschak
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@kortschak kortschak commented Jan 19, 2015

The documentation describing explicit argument indexes in format strings does not state that if an argument is refered to by an explicit argument index in the first place that following verbs must also have an explicit index.

Currently this gives surprising results:

package main

import "fmt"

func main() {
    fmt.Printf("%[2]d is the %[1]s\n", "answer", 42)
    fmt.Printf("%d is the %[2]s\n", 42, "answer")
    fmt.Printf("%[2]d is the %s\n", "answer", 42)
}
42 is the answer
42 is the answer
42 is the %!s(MISSING)

http://play.golang.org/p/HP11QHqa_p

@minux
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@minux minux commented Jan 19, 2015

“After processing a bracketed expression [n], arguments n+1, n+2, etc. will be processed unless otherwise directed.”

Working as intended.

@minux minux closed this Jan 19, 2015
@kortschak
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@kortschak kortschak commented Jan 19, 2015

The wording is less than entirely clear.

@minux
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@minux minux commented Jan 19, 2015

Then we have:

Because an explicit index affects subsequent verbs, this notation can be
used to print the same values multiple times by resetting the index for the
first argument to be repeated:

fmt.Sprintf("%d %d %#[1]x %#x", 16, 17)
will yield "16 17 0x10 0x11".

It states very clear that explicit index affects the following verbs, and
then you should
be able to understand the preceding statement:
"After processing a bracketed expression [n], arguments n+1, n+2, etc. will
be processed unless otherwise directed."

What confuses you?

@kortschak
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@kortschak kortschak commented Jan 19, 2015

What left me confused was that that final sentence does not explicitly mention that the argument cursor will be used as in the updated state (or words to that effect). I think a single clause between "processed" and "unless" making this clear would improve the explanation.

@minux
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@minux minux commented Jan 19, 2015

OK. Perhaps processing is confusing here. How about changing:
"After processing a bracketed expression [n], arguments n+1, n+2, etc. will be processed unless otherwise directed."
to:
"After processing a bracketed expression [n], subsequent verbs will use arguments n+1, n+2, etc. unless otherwise directed."

@minux minux reopened this Jan 19, 2015
@kortschak
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@kortschak kortschak commented Jan 19, 2015

SGTM

@minux minux closed this in e12b1dd Jan 19, 2015
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