Pipe Operator

Introduction

Programming can get messy. So messy in fact that function calls can get so embedded that they become difficult to follow. Take the following nested functions into consideration:

foo(bar(baz(new_function(other_function()))))

Here, we are passing the value other_function/0 to new_function/1, and new_function/1 to baz/1, baz/1 to bar/1, and finally the result of bar/1 to foo/1. Elixir takes a pragmatic approach to this syntactical chaos by giving us the pipe operator. The pipe operator which looks like |> takes the result of one expression, and passes it on. Let’s take another look at the code snippet above rewritten using the pipe operator.

other_function() |> new_function() |> baz() |> bar() |> foo()

The pipe takes the result on the left, and passes it to the right hand side.

Examples

For this set of examples, we will use Elixir’s String module.

• Tokenize String (loosely)

iex> "Elixir rocks" |> String.split()
["Elixir", "rocks"]

• Uppercase all the tokens

iex> "Elixir rocks" |> String.upcase() |> String.split()
["ELIXIR", "ROCKS"]

• Check ending

iex> "elixir" |> String.ends_with?("ixir")
true

Best Practices

If the arity of a function is more than 1, then make sure to use parentheses. This doesn’t matter much to Elixir, but it matters to other programmers who may misinterpret your code. It does matter with the pipe operator though. For example, if we take our third example, and remove the parentheses from String.ends_with?/2, we are met with the following warning.

iex> "elixir" |> String.ends_with? "ixir"
warning: parentheses are required when piping into a function call.
For example:

  foo 1 |> bar 2 |> baz 3

is ambiguous and should be written as

  foo(1) |> bar(2) |> baz(3)

true

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