How to Test for Valid Fields in Golang Templates Without Short-Circuiting?

Golang Template Testing for Valid Fields

Problem:

When attempting to test for the existence of a valid field within a Golang template using the and template function, several common methods have failed. This behavior stems from the fact that and is not short-circuit evaluated, unlike the && operator, causing errors like "invalid type for comparison" when attempting to compare a nil field to a value.

Solution:

Due to the non-short-circuit evaluation of and in templates, alternative approaches must be taken. Here are two methods that can be used for this scenario:

1. Nested if Statements:

{{if $.MyStruct.MyField}}
    {{if eq $.MyStruct.MyField.Value .}}selected="selected"{{end}}
{{end}}

In this approach, the outer if statement checks if $.MyStruct.MyField exists, and the inner if statement checks if its Value is equal to the desired value. This ensures that the comparison is only performed if the field is not nil.

2. with Action:

<select name="y">
   {{range $idx, $e := .SomeSlice}}
       <option value="{{.}}&quot; {{with $.MyStruct.MyField}}
               {{if eq .Value $e}}selected="selected"{{end}}
           {{end}}>{{.}}</option>
   {{end}}
</select>

The with action sets the dot to the provided value, allowing access to its fields. In this case, the with action is used to provide access to the $.MyStruct.MyField field within the inner scope, where the comparison is performed.

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