Pattern Matching
Like many other functional languages, Erlang has powerful pattern matching capabilities. Typically, a pattern (left side) is matched against a term (right side). Pattern matching can also occur in receive blocks, in which case the pattern is matched against the existing messages in a process queue. If the pattern match operation succeeds, then any unbound variables in the pattern will be bound. Pattern matching can be used in the following instances: 1) when evaluating a function 2) in a `case ... of` block 3) in a `receive` block when capturing the incoming messages to a process 4) in a `try` expression 5) when using the match operator '='
-module (patterns).
-compile([export_all]).
promote({ceo, male}) -> "man CEO now";
promote({ceo, female}) -> "woman CEO now";
promote({ceo, _}) -> "No CEO for you!".
demote(Arg) when is_tuple(Arg) ->
case Arg of
{ceo, male} -> "Fire the CEO";
{ceo, female} -> "Fire the CEO";
{ceo, _} -> "Cannot fire non existing CEO";
_ -> "unknown pattern"
end.
postman() ->
receive
{send, {Fr, To, _Content} = Pkg} ->
io:format("~s sending something to ~s ~n", [Fr, To]),
self() ! {recv, Pkg},
postman();
{recv, {To, Fr, Content} = _Pkg} ->
io:format("~s got a ~s from ~s~n", [Fr, Content, To]),
postman();
stop ->
io:format("Shutting down postman ...~n")
end.
is_even(Number) ->
Type = try Number rem 2 of
0 when is_number(Number) -> true;
1 when is_number(Number) -> false
catch
_ErrType:_Err -> "I can't tell"
end,
io:format("Is ~p even? ~p~n", [Number, Type]).
head(List) ->
[Head | _] = List,
io:format("Head of list is: ~p~n", [Head]).
run() ->
io:format("~p~n", [promote({ceo, female})]),
io:format("~p~n", [promote({ceo, male})]),
io:format("~p~n", [promote({ceo, other})]),
io:format("~p~n", [demote({ceo, kid})]),
Pid = spawn(?MODULE, postman, []),
Pid ! {send, {"Joe", "Jane", "cool gift"}},
timer:sleep(100),
Pid ! stop,
is_even(3),
head([101, 201, 301]).
1> patterns:run(). "woman CEO now" "man CEO now" "No CEO for you!" "Cannot fire non existing CEO" Joe sending something to Jane Jane got a cool gift from Joe Is 3 even? false Shutting down postman ... Head of list is: 101 ok
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