text/template: give position info on error

[bytbox]

Errors should include position info whenever possible, to make debugging templates less
reminiscent of tracking down segfaults without a debugger or core dump.

[bytbox]

Comment 1:

(This applies to html/template as well, of course.)

[dsymonds]

Comment 2:

Labels changed: added priority-later, packagechange, removed priority-triage.

Owner changed to @robpike.

Status changed to Accepted.

[robpike]

Comment 3:

I could do without the editorializing, but sure, this won't be too hard. The trick is to
integrate it well.

[moraes]

Comment 4:

I took a look at this and have a proposal to include filename (if available) and column
number for template errors. Example:
    _, err := template.New("foo").Parse(`
    
        {{if $foo}}{{end}}
    
    `)
Current error: 
    template: foo:3: undefined variable "$foo"
New error:
    template: foo:3:14 undefined variable "$foo"
For this, errorf now also accepts an argument for the input offset (taken from
item.pos), and the template filename is stored for debug purposes when available. My
plan is to remove lex.lineNumber() and rely only on item.pos for error messages.
Please let me know if it is acceptable:
    http://golang.org/cl/6442067
Note: I only implemented it in parse.useVar, so if you think it looks good I can extend
to the rest of the package.

[moraes]

Comment 5:

Updated with a full implementation: all errors display line/column numbers of the
related token and, when available, the filename.
http://golang.org/cl/6442067

[moraes]

Comment 6:

Not so fast. Exec errors still don't tell the position.
I'm thinking it'd be better to make lex.position() to return scanner.Position, and nodes
would store the position instead of simply the line number. Then execution errors would
have the full position info like parse errors.

[moraes]

Comment 7:

A new patch added position info to all exec errors as well. 
Because this replaces the Line field from nodes by a Pos field with scanner.Position,
this is not backwards compatible anymore (and broke html/template), I realized later.
I'll review this and make it fully compatible, keeping existing fields and using a
method for the position info.
Sorry for the comment spam. :P

[robpike]

Comment 8:

As explained in http://golang.org/doc/contribute.html, we prefer to
discuss significant changes by e-mail before sending code, let alone
submitting a code review through the issue reporting channel. Please
start a conversation in the regular forum to discuss what you'd like
to do. Once we have design we're happy with we can move to reviewing
code.
-rob

[moraes]

Comment 9:

Thanks. Coding some proof of concepts helped me to understand the problem and write a
proposal to improve this. I've sent it to the list now.

[rsc]

Comment 10:

Labels changed: added go1.1maybe.

[robpike]

Comment 11:

This issue was closed by revision 7f4b4c0.

Status changed to Fixed.