Erlang compound boolean expression

Question

I am still finding my feet with erlang, and I read the documentation about using "and", "or" operators, but why is the following not evaluating?

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

I am expecting for a "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

Without braces you can also do

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

Answer 3

You probably don't want to use and. There are two problems with and, firstly as you've noticed its precedence is strange, secondly it doesn't short circuit its second argument.

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

andalso was introduced later to the language and behaves in the manner which is probably more familiar. In general, use andalso unless you have a good reason to prefer and.

Btw, orelse is the equivalent of or.

Answer 4

herre's the operator precedence / binding rules table, for future reference

http://www.erlang.org/doc/reference%5Fmanual/expressions.html#id2274242

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here's some erlang "gotcha" lists i found helpful

http://stackoverflow.com/questions/430726/learning-erlang-speedbump-thread-common-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/