tim2CF
Purity validation
Hi again, I have question related to previous. To find answer I probably have to read compiler source code, but maybe you know answer already. I want some utility function which will do validation of ast, something like:
valid_guard?(ast) :: boolean
which I want to reuse to assume that given expression is pure. This function, or similar equivalent should exist in Erlang/Elixir compiler for sure, because it’s doing this purity validation well.
Writing own custom equivalent is not the best option, because you have to deal with possible things like
import Kernel, except: [{:'-', 1}]
import UnpureOverride, only: [{:'-', 1}]
But compiler is already doing all this smart stuff and can recognise this impurity
Marked As Solved
hauleth
erl_internal:guard_bif/2 or erl_lint:is_guard_test/1
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lpil
If you want to know if a macro is a valid guard you can use Macro — Elixir v1.20.2 and then traverse the AST, checking if all the calls in that AST are guards themselves.
If you wish to know whether a function it pure it is a much more difficult task, one not possible with the Elixir compiler today.
What do you mean by this? The Elixir compiler does not know whether functions are pure or not.
lpil
They don’t track if functions are pure, instead they have a small list of permitted functions and operators, and if any other functions are used then the code is rejected by the compiler. In Elixir it is the same process I outlined above with macro expansion.
Purity inference isn’t need as being pure isn’t enough for a function to be valid in a guard.
hauleth
No, not really. Erlang/Elixir just have defined set of “guard safe” functions which are allowed. There is much broader set of pure functions that will not be allowed in guards.
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