问题
Why does the Erlang if statement support only specific functions in its guard?
i.e -
ok(A) ->
if
whereis(abc)=:=undefined ->
register(abc,A);
true -> exit(already_registered)
end.
In this case we get an "illegal guard" error.
What would be the best practice to use function's return values as conditions?
回答1:
Coming from other programming languages, Erlang's if seems strangely restrictive, and in fact, isn't used very much, with most people opting to use case instead. The distinction between the two is that while case can test any expression, if can only use valid Guard Expressions.
As explained in the above link, Guard Expressions are limited to known functions that are guaranteed to be free of side-effects. There are a number of reasons for this, most of which boil down to code predictability and inspectability. For instance, since matching is done top-down, guard expressions that don't match will be executed until one is found that does. If those expressions had side-effects, it could easily lead to unpredictable and confusing outcomes during debugging. While you can still accomplish that with case expressions, if you see an if you can know there are no side effects being introduced in the test without needing to check.
One last, but important thing, is that guards have to terminate. If they did not, the reduction of a function call could go on forever, and as the scheduler is based around reductions, that would be very bad indeed, with little to go on when things went badly.
As a counter-example, you can starve the scheduler in Go for exactly this reason. Because Go's scheduler (like all micro-process schedulers) is co-operatively multitasked, it has to wait for a goroutine to yield before it can schedule another one. Much like in Erlang, it waits for a function to finish what it's currently doing before it can move on. The difference is that Erlang has no loop-alike. To accomplish looping, you recurse, which necessitates a function call/reduction, and allows a point for the scheduler to intervene. In Go, you have C-style loops, which do not require a function call in their body, so code akin to for { i = i+1 } will starve the scheduler. Not that such loops without function calls in their body are super-common, but this issue does exist.
On the contrary, in Erlang it's extremely difficult to do something like this without setting out to do so explicitly. But if guards contained code that didn't terminate, it would become trivial.
回答2:
Check this question: About the usage of "if" in Erlang language
In short:
Only a limited number of functions are allowed in guard sequences, and whereis is not one of them
Use case instead.
来源：https://stackoverflow.com/questions/48783536/why-does-erlang-if-statement-support-only-specific-functions-in-its-guard