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html/template: unexpected semantic differences compared to text/template #40075

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rsc opened this issue Jul 6, 2020 · 3 comments
Open

html/template: unexpected semantic differences compared to text/template #40075

rsc opened this issue Jul 6, 2020 · 3 comments
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@rsc
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@rsc rsc commented Jul 6, 2020

I ported the text/template tests over to html/template and discovered some semantic differences between the two that I did not expect.

In html/template:

  • Missing values format as empty strings, while in text/template they format as <no value>. For example:

    - 	{"map .NO", "{{.MSI.NO}}", "<no value>", tVal, true},
    + 	{"map .NO", "{{.MSI.NO}}", "", tVal, true}, // NOTE: <no value> in text/template
    
    - 	{"empty nil", "{{.Empty0}}", "<no value>", tVal, true},
    + 	{"empty nil", "{{.Empty0}}", "", tVal, true}, // NOTE: <no value> in text/template
    
    and so on
    
  • Missing values passed explicitly to the html escaper format as <nil> instead of <no value>:

    - 	{"html untyped nil", `{{html .Empty0}}`, "&lt;no value&gt;", tVal, true},
    + 	{"html untyped nil", `{{html .Empty0}}`, "&lt;nil&gt;", tVal, true}, // NOTE: "&lt;no value&gt;" in text/template
    
  • Given an addressable struct field with a pointer-receiver String method, the String method is not used (in text/template it is):

    - 	{"V{6666}.String()", "-{{.V0}}-", "-<6666>-", tVal, true},
    + 	{"V{6666}.String()", "-{{.V0}}-", "-{6666}-", tVal, true}, //  NOTE: -<6666>- in text/template
    
  • Given an addressable struct field with a pointer-receiver Error method, the Error method is not used (in text/template it is):

    - 	{"W{888}.Error()", "-{{.W0}}-", "-[888]-", tVal, true},
    + 	{"W{888}.Error()", "-{{.W0}}-", "-{888}-", tVal, true}, // NOTE: -[888] in text/template
    
  • TestExecuteOnNewTemplate crashes on all the cases from #11379.

  • TestMessageForExecuteEmpty shows a different error message:

    - template: empty: "empty" is an incomplete or empty template
    + template: "empty" is an incomplete or empty template
    
  • TestIssue31810 fails (“check that a parenthesized first argument behaves properly”, for #31810).

  • TestTemplateLookUp (for #10910, #10926) fails. Lookup returns non-nil values for undefined templates.

  • Calling Parse("") does not overwrite the text of a previous template parse, as it does in text/template.
    See TestEmptyTemplate, specifically the last case in the table:

    	{[]string{"{{.}}", ""}, "twice", ""},
    

The <no value> change looks like it may possibly be intentional. All the others look like bugs.

/cc @robpike

@rsc rsc added this to the Backlog milestone Jul 6, 2020
@gopherbot
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@gopherbot gopherbot commented Jul 6, 2020

Change https://golang.org/cl/241084 mentions this issue: html/template: add tests from text/template

gopherbot pushed a commit that referenced this issue Jul 14, 2020
Copy and adapt tests from text/template, to exercise more of html/template's copy.

Various differences in behavior are flagged with NOTE comments or t.Skip
and documented in #40075. Many of them are probably bugs.
One clarifying test case added to both text/template and html/template.

No changes to the package itself.

Change-Id: Ifefad83d647db846040d24c2741a0244b00ade82
Reviewed-on: https://go-review.googlesource.com/c/go/+/241084
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
@cfiderer
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@cfiderer cfiderer commented Sep 9, 2020

Hi Go experts,
is there any news on these behavioral differences?
We'd like to have missing values rendered as "<no value>" in HTML templates, too...

@rsc
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@rsc rsc commented Sep 10, 2020

@cfiderer My guess is that at this point adding <no value> to HTML templates would break Hugo users quite badly. /cc @bep

In general I think it would make sense to track down why these differences exist before removing them.

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