jonnifyelixir
F2 / F2.07 / product

F2.07 · part 1 of 3

Product types

A product type holds several values at once. A tuple bundles them by position; a struct bundles them by name. Either way every field is present together, and because each field varies independently, the number of distinct values is the product of the fields’ counts.

Tuples are quick and anonymous: {1, 2} is two values with no labels. Structs add names and a type: %Point{x: 1, y: 2} says these two numbers are a point. Both are immutable, so “changing” a field means building a new value with that field replaced.

Tuple or struct Updating a field Worked examples

Tuple or struct

The same pair of values can be a tuple or a struct. A tuple reads by position with elem/2; a struct reads by name with the dot. Names cost a little ceremony and buy clarity and a type tag.

Switch between the two encodings.

product type
several fields held together; its values are their product.
tuple
fields by position: {1, 2}, read with elem/2.
struct
fields by name with a type: %Point{x: 1, y: 2}.
field
one component of the product.

same data · positional or named

value {1, 2} index 0 1 index 1 2 read elem({1, 2}, 0) => 1 


      
tuple · fields by position · elem(t, 0) reads the first

    

    

      

        
Idea

        
Position or name — the same product, indexed two different ways.

      

      
      

        
Elixir

        
A struct is a map under the hood, tagged with __struct__.

      

    

    
Tuples index by position, structs by name. Reach for a struct when the fields deserve names and
      a type.

  


  
  

    
Updating a field

    

      
Structs are immutable, so updating a field does not change the original — it builds a new struct that
        shares the untouched fields. %{point | x: new} produces a moved point and leaves
        point exactly as it was.

      
Set the new x and compare.

    


    

      
%{point | x: new} · a new struct, original intact

      

        

          
          
          7
        

      


      
        point (original)
        
        %Point{
        x: 1, y: 2
        }
        moved (new)
        
        %Point{
        x: 7, y: 2
        }
      

      


      
moved has x = 7 · point is still x = 1, y = 2

    

    

      

        
F1.04 · immutability

        
Update returns a new value; shared fields are not copied, only referenced.

      

      
      

        
Elixir

        
The | update form requires the key to already exist on the struct.

      

    

    
Updating a struct builds a new one with the change applied; the original is untouched.