If you want to know if a macro is a valid guard you can use Macro — Elixir v1.16.0 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.

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.

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.

They do not have this functionality I’m afraid.

You’re effectively wanting to write a kind of static type system for Erlang- it would be a large piece of work.

I suspect it will not be, you will be limited to just these functions:

• !=/2
• !==/2
• */2
• +/1
• +/2
• -/1
• -/2
• //2
• <=/2
• ==/2
• ===/2
• >/2
• >=/2
• abs/1
• and/2
• binary_part/3
• bit_size/1
• byte_size/1
• ceil/1
• div/2
• elem/2
• floor/1
• hd/1
• in/2
• is_atom/1
• is_binary/1
• is_bitstring/1
• is_boolean/1
• is_float/1
• is_function/1
• is_function/2
• is_integer/1
• is_list/1
• is_map/1
• is_map_key/2
• is_nil/1
• is_number/1
• is_pid/1
• is_port/1
• is_reference/1
• is_struct/1
• is_tuple/1
• length/1
• map_size/1
• node/0
• node/1
• not/1
• or/2
• rem/2
• round/1
• self/0
• tl/1
• trunc/1
• tuple_size/1

There’s no way to iterate or recurse, and there is no flow control.