code review 5498048: template: better error message for empty templates

template: better error message for empty templates
New("x").ParseFiles("y") can result in an empty "x" template.
Make the message clearer that this is the problem. The error
returns from both template packages in this case were
confusing.

I considered making the method use "x" instead of "y" in
this case, but that just made other situations confusing
and harder to explain.

Fixes issue 2594.

[r, 2011-12-20 19:42:38]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://code.google.com/p/go

[rsc, 2011-12-20 19:44:10]

Test?

If I do New("x").Parse("") that's still okay, right?

[r2, 2011-12-20 19:45:37]

On Dec 20, 2011, at 11:44 AM, Russ Cox wrote:

> Test?
> 
> If I do New("x").Parse("") that's still okay, right?

This changes nothing but the error message.

-rob

[rsc, 2011-12-20 19:54:13]

On Tue, Dec 20, 2011 at 14:45, Rob 'Commander' Pike <r@google.com> wrote:
>> If I do New("x").Parse("") that's still okay, right?
>
> This changes nothing but the error message.

Sorry, I don't have enough context here to understand
your response.

I agree with having an error when Parse was never called.
I wanted to confirm that if you parse an empty string
that's still a valid template definition.  That is, if
    New("x").Parse("Hi")
followed by executing "x" is okay then I wanted to
make sure that
    New("x").Parse("")
can execute "x" too.

Russ

[r2, 2011-12-20 19:56:53]

On Dec 20, 2011, at 11:54 AM, Russ Cox wrote:

> On Tue, Dec 20, 2011 at 14:45, Rob 'Commander' Pike <r@google.com> wrote:
>>> If I do New("x").Parse("") that's still okay, right?
>> 
>> This changes nothing but the error message.
> 
> Sorry, I don't have enough context here to understand
> your response.
> 
> I agree with having an error when Parse was never called.
> I wanted to confirm that if you parse an empty string
> that's still a valid template definition.  That is, if
>    New("x").Parse("Hi")
> followed by executing "x" is okay then I wanted to
> make sure that
>    New("x").Parse("")
> can execute "x" too.

It is fine and was always fine to do New("x").Parse(""). This CL just changes
what string gets printed if you attempt to Execute a template that has never
been parsed, even to the empty string.  The word 'empty' is arguably confusing,
but 'undefined' seemed just wrong if you've said New("x").

-rob

[rsc, 2011-12-20 19:57:31]

LGTM

incomplete?

[r, 2011-12-20 20:58:26]

*** Submitted as d96df215f3b5 ***

template: better error message for empty templates
New("x").ParseFiles("y") can result in an empty "x" template.
Make the message clearer that this is the problem. The error
returns from both template packages in this case were
confusing.

I considered making the method use "x" instead of "y" in
this case, but that just made other situations confusing
and harder to explain.

Fixes issue 2594.

R=golang-dev, rsc, r
CC=golang-dev
http://codereview.appspot.com/5498048