Today I Learned

hashrocket A Hashrocket project

`hd` in Guard Tests

The hd method in Elixir returns the head of a list. It's kind of like List.first, but raises an error on an empty list.

iex(0)> hd [1,2,3]
iex(1)> hd []
** (ArgumentError) argument error

Browsing the docs, I learned something interesting: hd is allowed in guard tests.

Here's a contrived implementation of this behavior:

# example.ex
defmodule TestModule do
  def header(list) when hd(list) == 1, do: IO.puts "We did it"
iex(0)> TestModule.header([1,2,3])
We did it

When the guard's conditions aren't met, an error is raised:

iex(1)> TestModule.header([2,3,4])
** (FunctionClauseError) no function clause matching in TestModule.header/1
    iex:13: TestModule.header([2, 3, 4])

h/t Chris Erin & Micah Cooper

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