Pipe expressions

The pipe: expression type is an optional add-on to the TALES standard. Pipe expressions allow the transformation of values via a function delegate.

Pipe expressions are best used when a common transformation is required upon model values. One scenario which would be served well by pipe expressions is the formatting of dates/times.

Generally-speaking, pipe expressions are preferable to C# expressions for the execution of arbitrary logic. They are more performant and are easier to write.

Pipe expressions must be explicitly activated

Whilst support for pipe expressions is included in the main ZptSharp NuGet package, they are not one of the standard expression types. Pipe expressions must be explicitly activated using the method AddZptPipeExpressions() before they may be used.

Syntax

The syntax of a pipe expression is as follows:

pipe:variable delegate_expression

In this syntax, variable must be a single identifier for a defined variable. It is the input value which will be entered into the pipe delegate. The delegate_expression is any TALES expression that evaluates to a delegate.

Delegates

Valid delegates are those which match Func<TInput, TOutput>, where TInput and TOutput may be any types (except void). An attempt will be made to convert the input value to the appropriate type accepted by the delegate. An exception will be raised if the input value is of an incompatible type.

Output

The evaluated result of a pipe expression is the output of the delegate, when executed with the specified input.

Example

With a model that looks like the following:

{
  MyDate = DateTime.Today,
  FormatDate = (DateTime d) => d.ToString("dd MM yyyy")
}

The following source in a template document would render the current date as a space-separated day/month/year pattern.

<p tal:define="now here/MyDate">
  The date is
  <span tal:replace="pipe:now here/FormatDate">01 01 2000</span>.
</p>