Today I Learned

A Hashrocket project

Elixir with macro `<-` and `=`

So Elixir with macro accepts Matching clauses<- and Bare expressions=. Both match patterns and do not leak variables assigned inside these structures.

with {:ok, width} <- Map.fetch(%{width: 10}, :width),
  do: {:ok, 2 * width}
#=> {:ok, 20}

with {:ok, width} = Map.fetch(%{width: 10}, :width),
  do: {:ok, 2 * width}
#=> {:ok, 20}

So what is the difference between these?

When a match fails, matching clauses <- returns failed result but bare expressions = raises a MatchError:

with {:ok, width} <- Map.fetch(%{height: 10}, :width),
  do: {:ok, 2 * width}
#=> :error

with {:ok, width} = Map.fetch(%{height: 10}, :width),
  do: {:ok, 2 * width}
#=> ** (MatchError) no match of right hand side value: :error

Another difference is that when guard is only available for matching clause.

with {:ok, width} when is_number(width) <- Map.fetch(%{width: 10}, :width),
  do: {:ok, 2 * width}
#=> {:ok, 20}
Looking for help? Elixir is quickly gaining momentum for web applications that need concurrency, performance, and the ability to connect to many different clients. The developers at Hashrocket are learning along with the rest of the development community that Elixir and Phoenix are viable Rails alternatives for the right application. Check out the source code for Today I Learned, written in Elixir, and contact us if you need help with your Elixir project.