proposal: text/template: option for ignoring/returning errors in function calls

[blmayer]

The proposal is to add options like error=zero, error=default in a likewise fashion as in missingkey=....

The intention is to let users specify how function calls inside templates should be handled. In a template pipeline with functions (or methods) calls with 2 returns, a value and an error, execution stops if the error is not nil. I think it would be useful if the user could specify how to proceed. So passing options seems to be a natural way. For example:

• the option error=default keeps the same behavior: stop execution
• error=zero sets the returned value to be the zero value for that type and keeps executing
• error=errorvalue sets the returned value to be the error and keeps executing

Of course the key name could be different I just used error here to clarify. Implementing this would make error handling in templates possible, thus making it much more powerful. E.g.:

// Run function returns (string, error)
{{with .Run}}
{{if .}}
        no error case with error=zero or error value with error=errorvalue
        run is {{.}}
{{else}}
       error case, . is empty string on option `error=zero`
{{end}}
{{end}}

Thanks!

[seankhliao]

cc @robpike

[ianlancetaylor]

CC @robpike

[earthboundkid]

ISTM, you should wrap your functions in something that swallows the error if you want that behavior. For example,

// Run function returns (string, error)
// wrap converts it to (string)
{{with wrap .Run}}
{{if .}}
        no error case: {{.}}
{{else}}
       error case
{{end}}
{{end}}

[blmayer]

I already tried that using two versions:

1. wrap takes (string, error)
2. wrap takes string

Both cases fail with different errors:

1. executing "text.txt" at : wrong number of args for wrap: want 2 got 1
2. executing "text.txt" at <.Run>: error calling Run: [error from .Run]

I also tried more exotic stuff like a variadic wrap and using call inside the templates.

[seankhliao]

see #52437 for a different proposal on handling errors in templates

[blmayer]

Cool, that works for me too.