Capture variations within functions

Questions / Help
,
1

Hello and welcome to one of my first Elixir posts! Could someone please explain why the latter function, when ran, doesn’t return “Hello World!”? Does this have something to do with a “lambda function”? I have no idea what’s going on but I’m having fun. Thanks!

func = &IO.puts
func.("Hello World!")
 # :: Returns "Hello World!"

#This works
def talk(message), do: &IO.puts/1
msg = ModuleName.talk("Hello World!")
# :: Returns &IO.puts/1
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Welcome to Elixir @Earl

This is returning the capture IO.puts. The parameter message is unused in the above code.

You might have got a warning when the code was compiled.

warning: variable "message" is unused (if the variable is not meant to be used, prefix it with an underscore)
  iex:10: ModuleName.talk/1

We can remove the unused variable message from the code.
If you want to print Hello World, you will have to do something like this -

defmodule ModuleName do
  def talk(), do: &IO.puts/1
end
# then in iex  
iex> ModuleName.talk().("Hello World") 
Hello World
:ok

One other thing is IO.puts returns :ok - it writes “Hello World” to standard output.

Edit: added info about IO.puts return

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def talk(message), do: &IO.puts/1
msg = ModuleName.talk("Hello World!")

What is going on there is that you are returning &IO.put/1 but this anonymous function knows nothing about message

you should do something like this

def talk(message), do: fn -> IO.puts(message) end
msg = ModuleName.talk("Hello World!")

#and then apply msg (anonymous function of arity-0
msg.()
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4

One thing I’ve found really really important for folks learning a language: be very specific about the difference between “returning a value” and “printing a value”. Starting out in a REPL like iex can obscure this distinction, and it’s important to build a correct mental model.

IO.puts("Hello world") does two things:

  • it prints the string Hello world!
  • it returns the atom :ok
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5

Worth to mention IO.inspect inspects the value and prints it, AND returns said value, but it should never use as a means to return the value IMO.

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