Elemental expression Erlang boolean

Question

I still find my feet with erlang and I read the documentation about using the "and", "or" operators, but why doesn't the following evaluate?

X = 15, Y = 20, X==15 and Y==20. 

I expect "true" in the terminal, but I get a "syntax error before ==".

Answer 1

Try:

 X = 15. Y = 20. (X==15) and (Y==20). 

Answer 2

You probably don't want to use and. There are two problems, and, firstly, since you noticed that its priority is strange, and secondly, it does not close its second argument.

 1> false and exit(oops). ** exception exit: oops 2> false andalso exit(oops). false 

and was also introduced later into the language and behaves in a way that is probably more familiar. In general, use andalso if you have no good reason to prefer and.

Btw, orelse is the equivalent of or.

Answer 3

Without braces you can also do

 X = 15. Y = 20. X == 15 andalso Y == 20. 

Answer 4

herre operator priority / binding rules table for future reference

http://www.erlang.org/doc/reference_manual/expressions.html#id2274242

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here are some "gotcha" erlang lists that I found useful

Exploring Erlang? head speed, general, small problems

http://www.erlang.org/faq/problems.html

http://baphled.wordpress.com/2009/01/05/lighting-up-the-tunnerl-pt-3-afterl-the-basics/

http://baphled.wordpress.com/2009/03/13/lighting-up-the-tunnerl-pt-9-the-gotchaz/