PatternMatching

= Pattern Matching =

On 2015-07-10 in #elixir-lang I asked

8:22 AM I get def zzz(conn, params) do - easy, match when the function is called with two params, I don’t care what they are

8:23 AM and zzz(_conn, %{a: myvar}) matches when the second param is a map that has :a as a key, and binds myvar to the value stored in the map under the key :a

8:24 AM wsmoak, rossjones: That is pattern matching, its equivalent in that case if you do params = %{...} or %{...} = params - its more obvious in the second case that it's pattern matching

8:25 AM micmus: thanks! but... def zzz(_conn, params = %{b: myvar}) do ??? I know it matches when the second param is a map with a key of :b (as above). but I'm missing the vocabulary to describe... and also binds params to the value of ... the entire map

8:25 AM how does a value being passed in "hit" both sides of that equals statement?

8:26 AM wsmoak: = is the pattern match assertion, not assignment...if both sides are true, the semantics are preserved

8:26 AM wsmoak: Its easier to look at pattern matching differently. It's looking at that expression and checking if it can be made true - yes it can - by binding params to entire map, and myvar to value of key b

8:27 AM It's like if you did params = %{b: myvar} = the_argument

8:28 AM where the_argument is the passed value

8:28 AM (a = %{foo: "bar"}) = %{foo: "bar", baz: "quux"}

8:31 AM I may have to miss work today. I need time to absorb this. :)

8:37 AM thanks asonge and micmus. very helpful in shifting the way I'm thinking about the syntax.