How Do Pipelines and the Dot (.) Operator Work in Go Templates?

Golang Template Engine and Pipelines

Understanding Pipelines in Golang Templates

In Golang templating, pipelines refer to a series of commands that manipulate and transform data. These commands can include basic values, function calls, or method invocations. A pipeline may contain multiple commands, separated by the | character. The output of each command is passed as the last argument to the next command in the pipeline.

The Role of "dot" (.) in Pipelines

The "dot" (.) is a cursor that points to the current position in the data structure used by the template. When you execute a template, the dot is initially set to the value passed to the template execution. As the template is processed, the dot may change its value to point to specific fields or substructures within the data.

Pipelines in Template Invocations

When invoking another template using the {{template}} function, you can specify the data value to be used within the invoked template by using the dot as the pipeline argument. For example, {{template "myTemplate" .}} passes the current dot value to the template named "myTemplate".

Using "dot" and "$" for Data Access

While the dot cursor provides access to the current value within the template, the $ symbol can be used to access the original value passed to the template execution. This is useful when the dot has been modified by actions like {{range}} or {{with}}.

Example

Consider the following template:

{{- define "test" -}}
{{- printf "%s" .Name | trunc 24 -}}
{{- end -}}

In this example, the dot (.Name) refers to the Name field of the data structure passed to the template execution. The | trunc 24 pipeline applies the trunc function to the Name value, truncating it to 24 characters. This transformed value is then printed using printf.

Conclusion

Pipelines and the "dot" are fundamental concepts in Golang templating. They provide a powerful mechanism for manipulating and accessing data within templates, allowing for flexible and dynamic template generation.

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