is_time, is_date and is_naive_datetime macros ?
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João Bernardo
Oct 25, 2016, 3:29:58 PM10/25/16
to elixir-lang-core
Hi !
Why aren't there is_time, is_date and is_naive_datetime guards ?
For sure I am not the first one to propose this, and baybe there is a reason for that.
But I believe this would be really useful.
Thanks.
José Valim
Oct 25, 2016, 3:34:35 PM10/25/16
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Unfortunately we cannot check structs on guards. Fortunately we can pattern match on them:
def my_function(%Time{}) do ...
def my_function(%Date{}) do ...
etc.
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João Bernardo
Oct 25, 2016, 3:51:37 PM10/25/16
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Hum... I am new to elixir, why can't we check structs on guards ? Writing a macro to check a struct seems easy.
Nonetheless, pattern match does the job.
terça-feira, 25 de Outubro de 2016 às 20:34:35 UTC+1, José Valim escreveu:
Unfortunately we cannot check structs on guards. Fortunately we can pattern match on them:def my_function(%Time{}) do ...def my_function(%Date{}) do ...etc.
On Tue, Oct 25, 2016 at 9:29 PM, João Bernardo <jb.am...@gmail.com> wrote:
Hi !Why aren't there is_time, is_date and is_naive_datetime guards ?For sure I am not the first one to propose this, and baybe there is a reason for that.But I believe this would be really useful.Thanks.
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Onorio Catenacci
Oct 25, 2016, 4:02:09 PM10/25/16
to elixir-lang-core
It's a limitation of the underlying BEAM machine. Only certain expressions are valid in guards.
From this page: http://erlang.org/doc/reference_manual/expressions.html#id2273702
"The reason for restricting the set of valid expressions is that evaluation of a guard expression must be guaranteed to be free of side effects."
If there is a way to guarantee that any given macro is certain to be free of side-effects then it could be supported although it'd require a change to the underlying BEAM itself.
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Onorio
João Bernardo
Oct 25, 2016, 4:11:43 PM10/25/16
to elixir-lang-core
Makes sense.
Thanks for the answer !
Vasilis Spilka
Apr 15, 2017, 7:08:02 AM4/15/17
to elixir-lang-core
While we cannot guard directly on the struct we can `hack` it a bit and do something like this
```
```
@dates [NaiveDateTime, DateTime]
def funky(%{__struct__: struct} = map) when struct in @dates, do: IO.puts "I am some date :D !"
```
def funky(%{__struct__: struct} = map) when struct in @dates, do: IO.puts "I am some date :D !"
```
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