Piping - pf-lang/pipefish GitHub Wiki

This page explains the use of the ->, >>, and ?> operators.

The piping operator ->

The -> operator takes a value, on the left, and insets it into a function, on the right. So for example 5 -> triangularNumber is just the same as triangularNumber 5.

This is useful because it allows us to talk about operations on data from left to right. An expression like "hElLo wOrlD" -> toLowerCase -> reverse -> capitalizeInitials -> reverse is easier to read, and for you to evaluate as "hellO worlD", than if it was written as reverse capitalizeInitials reverse toLowerCase "hElLo wOrlD".

Instead of a function, we can write an expression which uses the word that: for example 3 -> that + 5 -> that * that will evaluate to 64.

The mapping operator >>

This takes a list on the left and a function on the right, and returns the result of applying the function to each element of the list. So ["fee" "fie", "fo", "fum"] >> len would evaluate to [3, 3, 2, 3].

Again, we can use the that keyword to write mapping expressions: [1, 2, 3, 4, 5] >> that + 5 >> that * that would evaluate to [36, 49, 64, 81, 100].

The filter operator ?>

This takes a list on the left and a function returning a boolean value on the right, and returns those elements for which the application of the function to the element returns true. So for example [1, 2, 3, 4, 5] ?> that % 2 == 1 will select the odd elements and return [1, 3, 5].