[erlang-questions] Don't understand the error message: call to local/imported function ... is illegal in guard

[by]

Only limited expressions are permitted after “if”(guard), and the user defined function does not in the list.
I believe there is a list on Erlang Doc about what is permitted.

By the way, this limitation is for avoiding side effects when evaluating expressions.

Yao

> 在 2018年12月24日，08:26，Donald Steven <t6sn7gt@REDACTED> 写道：
> 
> This code works:
> 
>     AlphaNumericChar = isAlphaNumeric(C),
>     if AlphaNumericChar ->
> 
> but this code (which is functionally identical):
> 
>     if isAlphaNumeric(C) ->
> 
> produces an error message: call to local/imported function isAlphaNumeric/1 is illegal in guard
> 
> Why?
> 
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