proposal: text/template: export error type for missingkey

[sding3]

There is currently not an easy way to test for missing key error from text/template in the case that that the template was constructed with the missingkey=error option.

I propose adding a new error type in text/template testable via errors.As and the missing key can be recovered:

type MissingKeyError struct {
    Key string
}

func (e MissingKeyError) Error() string {
    return "no entry for key"
}

This would be added in a way that doesn't cause the existing error's string representation to change, allowing any users that might be checking for certain sub-string in the error strings to gracefully transition to the new way.

---

Edit: changed fmt.Errorf to errors.New Edit: took suggestion from @icholy to have the error be a struct type, allowing users to be able to recover the offending key.

[seankhliao]

cc @robpike

[icholy]

Perhaps something like:

type MissingKeyError struct {
    Key string
}

func (e MissingKeyError) Error() string {
    return "no entry for key"
}

[sding3]

@icholy , great idea! I've updated the proposal.

[sethvargo]

It looks like this might already exist (except for write errors): https://github.com/golang/go/blob/master/src/text/template/exec.go#L118-L124

[sding3]

It looks like this might already exist (except for write errors): https://github.com/golang/go/blob/master/src/text/template/exec.go#L118-L124

Can you show an example on programmatically detecting whether the cause for an ExecError is due to missing keys? I don't think that capability exists today unless you resort to sub-string checks against the error string, which isn't a reliable method.