[erlang-questions] illegal guard expression for IF illegal guard expression for "if"

[Richard O'Keefe]

On 5/12/2011, at 3:57 PM, Daniel Dormont wrote:

> In my experience with Erlang so far, this is perhaps the weirdest
> feature for me from a language standpoint. I understand the
> motivations, but at least from where I sit, it's a weird mental leap
> from there to "we allow these few functions, but not others."

In old Erlang, it was a lot clearer: the things that were allowed in
guards were almost all things that were not allowed anywhere else.
For example X < Y was allowed as a guard but not as an expression.

To this day, I stick with "," and ";" in guards, reserving
"andalso" and "orelse" for expressions (and trying to avoid those).

> In fact,
> not even all of the allowed functions are "pure" in the sense that,
> say, a Haskell programmer would recognize. I'm specifically thinking
> of node/0 and self/0.

self() cannot change in a process.
I don't believe node() can change either, but I could be wrong.

> What are abstract patterns?

Functions are abstractions of expressions.
Abstract patterns are abstractions of patterns.

For example, suppose you often want to match [{opt,X}].
Then you will one day be able to write

	#one_opt(X) -> [{opt,X}].

and then use #one_opt(X) wherever you would have written the pattern.
The proposal has been around for years, but the OTP team have always
had more urgent maddened grizzly bears to stun.