I have the following code. I am checking 3 conditions. You can see for the first condition I stored the output of xml:get_tag_attr_s(...)
in a variable and then used the variable within the if block. My problem is I get error illegal guard expression
, if I try to do the above process in one line like I did for the other two conditions.
Also, I am getting variable '_' is unbound
from the default condition. It supposed to be the same thing.
Can somebody please explain the issue?
validate_xmpp(Packet) ->
Type = xml:get_tag_attr_s(list_to_binary("type"), Packet),
if
(Type /= <<"chat">> ->
{error, "Message type is not chat"};
xml:get_path_s(Packet, [{elem, list_to_binary("body")}, cdata]) /= <<"">> ->
{error, "No or empty body"};
exml_query:path(Packet, [{element,<<"received">>},{attr,<<"xmlns">>}]) == <<"urn:xmpp:receipts">> ->
{error, "delivery-receipts should be ignored"};
_->
{ok, xml:get_tag_attr_s(list_to_binary("from"), Packet)}
end.
Erlang allows only these to be guards:
true
andalso
and orelse
)For more info take a look http://www.erlang.org/doc/reference_manual/expressions.html#id83606
Instead of _
use true
. You cannot use _
in if
, only in case
statements, and also take a look at the docs.
isPrime(A,B) when B>math:sqrt(A) -> true;
That results in an illegal guard error.
On a first reading, it looks like the guard contains a "term comparison":
>
and an "arithmetic expression":
math:sqrt(A)
Futhermore, if you play around with the code, you will see that the guard:
B > A+2
is legal. So what's the difference between the "arithmetic expression" math:sqrt(A)
and A+2
?
The Erlang docs define an "arithmetic expression" as: `
+
-
*
/
div
rem
bnot
band
bor
bxor
bsl
bsr
Notably, math:sqrt()
is not in the list of "arithmetic expressions". Therefore, math:sqrt(A)
is a "function call" rather than an "arithmetic expression", and you can only call a certain limited number of functions in a guard, namely the "type test BIF's" listed here, such as:
is_integer/1
is_float/1
is_binary/1
is_list/1
is_map/1
is_function/1
etc.
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