Erlang Pattern Matching — The = Operator, Guards, and Function Clauses

%% In Erlang, = is the match operator, NOT assignment
X = 5.     %% Binds X to 5 (first time only)
X = 5.     %% OK: matches (5 = 5)
% X = 6.   %% Error: no match (5 ≠ 6)

%% Matches must succeed
{Name, Age} = {alice, 30}.   %% Name => alice, Age => 30
{Name, 30} = {bob, 30}.     %% Name => bob (matches age)
% {Name, 30} = {bob, 25}.   %% MatchError!

%% Variables can only be bound once (in the same scope)
X = 10.
% X = 20.  %% Error: X is already bound

%% But rebinding is allowed in different clauses
case Something of
    1 -> X = 10;
    2 -> X = 20
end.

%% Tuples
{ok, Content} = file:read_file("exists.txt").
{error, enoent} = file:read_file("missing.txt").

%% Lists
[First, Second | Rest] = [1, 2, 3, 4, 5].
First.   %% => 1
Second.  %% => 2
Rest.    %% => [3, 4, 5]

%% Maps
#{name := N, age := A} = #{name => alice, age => 30}.

%% Guards extend patterns with conditions
factorial(0) -> 1;
factorial(N) when N > 0 -> N * factorial(N - 1).

%% Multiple guard conditions
describe_temp(T) when T < 0 -> freezing;
describe_temp(T) when T >= 0, T < 15 -> cold;
describe_temp(T) when T >= 15, T < 25 -> warm;
describe_temp(T) when T >= 25, T < 35 -> hot;
describe_temp(_) -> very_hot.

%% Guard functions: is_atom/1, is_integer/1, is_list/1, etc.
process(X) when is_integer(X) -> integer;
process(X) when is_float(X) -> float;
process(X) when is_list(X) -> list.

%% Multiple clauses are tried in order
-module(math).
-export([fib/1]).

fib(0) -> 0;
fib(1) -> 1;
fib(N) when N > 0 -> fib(N-1) + fib(N-2).

%% Pattern matching on arguments
area({circle, R}) -> math:pi() * R * R;
area({rectangle, W, H}) -> W * H;
area({triangle, B, H}) -> B * H / 2.

%% case is pattern matching in expression form
case file:read_file("test.txt") of
    {ok, Content} ->
        io:format("Read ~p bytes~n", [byte_size(Content)]);
    {error, enoent} ->
        io:format("File not found~n");
    {error, Reason} ->
        io:format("Error: ~p~n", [Reason])
end.