erlang "illegal guard expression" while using function in Guards

Erlang allows only these to be guards:

• The atom true
• Other constants (terms and bound variables), all regarded as false
• Calls to the BIFs (built-in functions) specified in table Type Test BIFs
• Term comparisons
• Arithmetic expressions
• Boolean expressions
• Short-circuit expressions (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.