Legal guards in Erlang are boolean functions placed after the key word, "when" and before the arrow, ">". Guards may appear as part of a function definition, 'receive' 'if', 'case' and 'try/catch' expressions.
We can use a guard in a function definition
Example program: guardian.erl
module(guardian). compile(export_all). the_answer_is(N) when N == 42 > true; the_answer_is(N) > false. % ============================================= >% % % Example output: % % c(guardian). % ok % % guardian:the_answer_is(42). % true % % guardian:the_answer_is(21). % false
and Fun definition
F = fun(N) when N == 42 > true; (N) > false end.
receive expression
receive {answer, N} when N == 42 > true; {answer, N} > false end.
if expression
if N == 42 > true; true > false end.
case expression
case L of {answer, N} when N == 42 > true; _ > false end.
and try/catch
try find(L) of {answer, N} when N == 42 > true; _ > false catch {notanumber, R} when is_list(R) > alist; {notanumber, R} when is_float(R) > afloat _ > noidea end.
You will notice that in these examples it would be clearer to remove the guard and modify the pattern matching instead.
Literate programming note: Anonymous match variables that start with an underscore like "_" are not generally recommended. Rather, it is nice to use some descriptive variable name like "_AnyNode". On the otherhand, for tutorial code like this, a descriptive variable is more distracting than helpful.
case L of {node, N} when N == 42 > true; _AnyNode > false end.
There are several builtinfunctions (BIFs) which may be used in a guard. Basically we are limited to checking the type with, is_type(A) and the length of some types with, type_size() or length(L) for a list length.
is_alive/0 is_boolean/1 is_builtin/3 is_constant/1 is_float/1 is_function/2 is_function( Z, Arity) is_function/1 is_integer/1 is_list/1 is_number/1 is_pid/1 is_port/1 is_record/3 is_record/2 is_reference/1 is_tuple/1 tuple_size/1 is_binary/1 is_bitstring/1 bit_size/1 byte_size/1 length(Z) > N
A > B A < B A == B A =< B A >= B A /= B A =:= B exactly equal A =/= B exactly not equal
Note: all erlang data types have a natural sort order.
atom < reference < port < pid < tuple < list ...
Guards can be compound expressions.
the_answer_is(N) when N == 42, is_integer(N) > true;
