jonnifyelixir
F2 / F2.06 / capture

F2.06 · part 2 of 3

The capture operator

The capture operator & writes a small anonymous function in one compact form. Inside &(…), the placeholders &1, &2, and so on stand for the arguments. It also captures a named function directly, as &Module.function/arity.

There are two distinct uses sharing the symbol. The first wraps an expression: &(&1 + 1) is the same function as fn x -> x + 1 end. The second names an existing function: &String.upcase/1 is the function String.upcase itself, ready to pass along. Both produce ordinary functions.

Positional placeholders Capturing named functions In the wild

Positional placeholders

Wrap an expression in &(…) and use &1 for the first argument, &2 for the second. The highest placeholder sets the arity. The result is identical to the matching fn.

Compare each shorthand with its full form.

capture operator
& — builds a function from an expression or a name.
placeholder
&1, &2 … the positional arguments.
function capture
&Module.fun/arity — the named function as a value.
arity
the number of arguments — set by the highest placeholder.

& form and fn form · same function

& form &(&1 + 1) ≡ fn form fn x -> x + 1 end arity 1 


      
&(&1 + 1) · arity 1 · &1 is the first argument

    

    

      

        
Idea

        
The placeholders are the parameters; the highest one fixes how many there are.

      

      
      

        
Elixir

        
One nuance: &(&1) on its own is not allowed — the body must be a call.

      

    

    
The & form is a literal fn with numbered parameters. Reach for it when
      the body is a single short expression.

  


  
  

    
Capturing named functions

    

      
The same operator turns a named function into a value. &String.upcase/1 is the
        function itself — no wrapper, no placeholders — ready to hand to Enum.map or
        store in a variable. The arity says which function of that name.

      
Pick a captured function and see it applied.

    


    

      
&Module.fun/arity · a named function as a value

      

        

          
          
          
          
        

      


      
        capture
        
        &String.upcase/1
        ≡
        fn form
        
        fn s -> String.upcase(s) end
        apply to "ab"  =>  "AB"
      

      


      
&String.upcase/1 · the function itself, ready to pass · "ab" → "AB"

    

    

      

        
F1.07 · higher-order

        
Capturing names is the tidiest way to pass an existing function as an argument.

      

      
      

        
Elixir

        
The /arity is required — it picks among functions sharing a name.

      

    

    
Function capture passes a named function without wrapping it. The arity selects which one.