I afraid you missed my point, I might have expressed it poorly. Let's
assume I have a simple type: {is_atom(), is_number(), is_binary()}. I want
to define a guard to match it. Without reusing I can write a function
accepting it:
func({x, y, z}) when is_atom(x) and is_number(y) and is_binary(z), do: true
but then I want to define another function which expects the same tuple:
another({x, y, z}) when is_atom(x) and is_number(y) and is_binary(z), do:
true
I don't have a way to define a custom guard to match tuple elements since
there is no pattern matching in defguard nor there is `elem` in guards. So
both options don't work:
defguard is_mytype({x, y, z}) when is_atom(x) and is_number(y) and
is_binary(z)
nor
defguard is_mytype(x) when is_atom(elem(x, 0)) and is_number(elem(x, 1))
and is_binary(elem(x, 2))
Furthermore, I would want to define a function that receives a value of my
type inside of complex structure:
function({:ok, {x, y, z}}) when is_atom(x) and is_number(y) and
is_binary(z), do: true
it would be cool to have it defined as
function({:ok, x}) when is_mytype(x), do: true
P.S. Actually, I've found that `elem` works in guards, so I can define my
guard without pattern matching. That's good for now, but
func({x, y, z}) when is_atom(x) and is_number(y) and is_binary(z), do: true
sounds cooler, IMHO =)
On Wednesday, November 7, 2018 at 8:20:22 PM UTC+2, Louis Pilfold wrote:
>
> Hi Sergiy
>
> The functionality you've described can be implemented with macros, no need
> to modify Elixir or Erlang.
>
> To start it could be as simple as defining guards that assert nothing in
> the production environment.
>
> defmodule Test do
> if Mix.env() == :prod do
> defguard is_my_type(x) when true
> else
> defguard is_my_type(x) when is_atom(x)
> end
>
> def go(x) when is_my_type(x) do
> x
> end
> end
>
> This could be a little error prone though as unless you remember to apply
> the guard to every clause of the function your logic may change when they
> are removed. Even if you apply them to every clause if you use exceptions
> as flow control you may run into problems as values that previously would
> result in a FunctionClauseError would be passed though.
>
> Plenty to think about! Perhaps experiment with a little proof of concept
> library and see what happens :)
>
> Cheers,
> Louis
>
> On Wed, 7 Nov 2018 at 17:44 Sergiy Kukunin <[email protected]
> <javascript:>> wrote:
>
>> Thanks for the answers. Just want to note, that I don't want to invent
>> type system such as in statically typed languages. I mean more about
>> defining schemas we can check different values with. All pattern matching,
>> guards and typespec might work for this. Furthermore, it would be cool to
>> make it composable and reusable (such as defguards and typespecs right now).
>>
>> Just to conclude, I would suggest that either of these would improve the
>> safety and convenience of the language:
>> - allow pattern matching in custom guards (either via the built-in guard
>> such as `Kernel.match?/2` or by extending the defguard syntax)
>> - having a macro to check whether a value corresponds to a defined @type
>>
>> What's about such syntax?
>>
>> defguard is_mytype({x, y}) when is_atom(x) and is_number(y)
>>
>> def test({:ok, value}) when is_mytype(value), do: true
>> def test(_), do: false
>>
>> test({:ok, {:hello, 5}}) # should be true
>> test({:ok, {2, 5}}) # should be false
>>
>> There are a couple of reasons I've raised this question:
>>
>> - do I miss something? don't I try to solve the problem in a wrong way?
>> - to estimate how hard is it to implement in a 3rd-party library or does
>> it require changes to core Elixir/ErlangVM
>>
>> Thanks
>>
>>
>> On Wednesday, November 7, 2018 at 7:20:47 PM UTC+2, Louis Pilfold wrote:
>>
>>> Hi all
>>>
>>> The desire for more safety in Elixir is reasonable, both at compile time
>>> and at runtime.
>>>
>>> The core team have previously experimented with introducting a compile
>>> time type checking system, and we also have the dialyser and gradualizer
>>> tools that can be used with Elixir.
>>>
>>> Checking at runtime is something we already do in Elixir and Erlang
>>> through the use of pattern matching and guards such as `is_binary/1`.
>>> A library of macros that automates these checks could be an interesting
>>> project, perhaps an area worth exploring for members of the community.
>>>
>>> Cheers,
>>>
>> Louis
>>>
>>> On Wed, 7 Nov 2018, 16:46 Ivan Yurov, <[email protected]> wrote:
>>>
>> If you want type-safety why not to just pick a strongly typed language,
>>>> like Ocaml for example? Elixir is bound to Erlang VM and will never
>>>> provide
>>>> any features like you're describing that are not supported by Erlang. And
>>>> I
>>>> don't think type-checking ever happens at runtime in any language.
>>>>
>>>> On Wednesday, November 7, 2018 at 12:00:53 PM UTC+1, Sergiy Kukunin
>>>> wrote:
>>>>>
>>>>> Hello there. This is my first message to the elixir group. Thanks for
>>>>> the great language.
>>>>>
>>>>> While I'm writing my code, I want to make functions to be safer. It's
>>>>> bad practice if a function accepts unexpected input and pass it further,
>>>>> and it blows in a completely different part of a system.
>>>>>
>>>>> At first glance, I have pattern matching, but it's pretty limited. It
>>>>> becomes really powerful in conjunction with guards, so I can write a
>>>>> signature to match literally everything.
>>>>> But they hard to re-use, If I have multiple functions operating with
>>>>> the same object. Yes, I can define a custom guard, but can I use pattern
>>>>> matching there? `Kernel.match?/2` doesn't work, so I'm limited with only
>>>>> guards in my custom guards.
>>>>>
>>>>> Another thing that we have typespecs. It seems exactly what I'm
>>>>> looking for: you have a wide set of built-in types, and I can easily
>>>>> compose and reuse my own types. The problem with it, that it doesn't
>>>>> affect
>>>>> runtime. I know about static analyzer `dialyzer`, but I'm not sure it
>>>>> will
>>>>> catch all cases since it's a static check, not a runtime.
>>>>>
>>>>> Let's assume a simple function, that wraps a value into a list:
>>>>>
>>>>> @spec same(number()) :: [number()]
>>>>> def same(number) do
>>>>> [number]
>>>>> end
>>>>>
>>>>> I'm sure the `dialyzer` won't complain since a signature is valid. But
>>>>> what if I do: `same("abc")` ? What will prevent Elixir from returning a
>>>>> wrong type? I guess, nothing.
>>>>> An example from a real life: I have a function, that accepts a custom
>>>>> shaped value (using tuples) and feeds it to a queue. Then, in the totally
>>>>> different part of the system, a consumer gets values from the queue. And
>>>>> when a wrong value was fed on the producer side, it blows on the consumer
>>>>> side. So I decided to put some constraints on the producer side to fail
>>>>> fast.
>>>>>
>>>>> Yes, I could define a guard, but again, if I have a pretty complex
>>>>> type instead of the simple `number`, I had to duplicate the type
>>>>> defining:
>>>>> one for typespec, another is for a custom guard (which is limited, since
>>>>> I
>>>>> can't use pattern matching there).
>>>>>
>>>>> Wouldn't it be cool, If we had a mechanism to assert a value to its
>>>>> type, in runtime? To avoid performance penalty we could enable it only
>>>>> for
>>>>> runtime. Is there a way right now to check whether a value corresponds to
>>>>> a
>>>>> type in runtime? Can I implement a custom macro to provide a good DSL for
>>>>> this? Is it helpful at all?
>>>>>
>>>>> P.S. You may say, use structs and pattern matching would work in this
>>>>> case. But what if my type is better represented by a tuple: {atom(),
>>>>> pos_integer(), string()}. Converting it to a struct might complicate a
>>>>> way
>>>>> to work with the value.
>>>>>
>>>> --
>>>> You received this message because you are subscribed to the Google
>>>> Groups "elixir-lang-core" group.
>>>>
>>> To unsubscribe from this group and stop receiving emails from it, send
>>>> an email to [email protected].
>>>
>>>
>>>> To view this discussion on the web visit
>>>> https://groups.google.com/d/msgid/elixir-lang-core/8c4d9dac-134d-471c-a402-e9696bf5aecf%40googlegroups.com
>>>>
>>>> <https://groups.google.com/d/msgid/elixir-lang-core/8c4d9dac-134d-471c-a402-e9696bf5aecf%40googlegroups.com?utm_medium=email&utm_source=footer>
>>>> .
>>>> For more options, visit https://groups.google.com/d/optout.
>>>>
>>> --
>> You received this message because you are subscribed to the Google Groups
>> "elixir-lang-core" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to [email protected] <javascript:>.
>> To view this discussion on the web visit
>> https://groups.google.com/d/msgid/elixir-lang-core/c7e602a5-a694-46f9-99a5-983b4d50eea0%40googlegroups.com
>>
>> <https://groups.google.com/d/msgid/elixir-lang-core/c7e602a5-a694-46f9-99a5-983b4d50eea0%40googlegroups.com?utm_medium=email&utm_source=footer>
>> .
>> For more options, visit https://groups.google.com/d/optout.
>>
>
--
You received this message because you are subscribed to the Google Groups
"elixir-lang-core" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/elixir-lang-core/f6a0f326-ffa4-4b69-998d-6f60a91abe87%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
