Thanks for the explanation about guards, makes sense. And I totally agree
with you, that can be implemented as an assertion at the very begin of
function's body.
Again, the question - is there a way to leverage typespecs? Is there a way
to implement something like this "hey elixir, is this value corresponds to
this type?"
I'd like to avoid a custom way to define specs for that assertions macros.
On Thu, Nov 8, 2018 at 12:58 AM Louis Pilfold <lo...@lpil.uk> wrote:
> Hey
>
> The implementation you've given there is expensive and only works for
> lists up to a certain length.
>
> To solve this one you'll need to step outside of guard clauses as they
> only support a limited subset of Elixir/Erlang. The idea is that all
> operations in guards are very fast and run in constant time, so iterating
> over a list or arbitrary length is not supported.
>
> Another option would be to write a macro that prepends a type checking
> statement to a function body, asserting that the arguements are of the
> correct type.
>
> Cheers,
> Louis
>
> On Wed, 7 Nov 2018, 22:41 Sergiy Kukunin, <sergey.kuku...@gmail.com>
> wrote:
>
>> Found another problem: can't express "list of strings" in guards nor
>> pattern matching. It's an easy task for typespecs `[String.t(), ...]`, but
>> I can't check typespecs in runtime. Found a very dirty hack, that works for
>> lists up to 5 strings, enjoy:
>>
>> defguardp is_list_of_strings(x)
>> when (length(x) == 1 and is_binary(hd(x)))
>> or (length(x) == 2 and is_binary(hd(x)) and is_binary(hd(tl(x))))
>> or (length(x) == 3 and is_binary(hd(x)) and is_binary(hd(tl(x))) and
>> is_binary(hd(tl(tl(x)))))
>> or (length(x) == 4 and is_binary(hd(x)) and is_binary(hd(tl(x))) and
>> is_binary(hd(tl(tl(x))))
>> and is_binary(hd(tl(tl(tl(x))))))
>> or (length(x) == 5 and is_binary(hd(x)) and is_binary(hd(tl(x))) and
>> is_binary(hd(tl(tl(x))))
>> and is_binary(hd(tl(tl(tl(x))))) and
>> is_binary(hd(tl(tl(tl(tl(x)))))))
>>
>> Wouldn't it be cool to be able to write something like
>>
>> defguard is_list_of_strings(x) match_type([String.t(), ...])
>>
>> Again, I'm pretty new, and I know nothing about the implementation and
>> where Elixir ends and Erlang starts, and how feasible it is. Just an idea
>> =)
>>
>> On Wed, Nov 7, 2018 at 10:11 PM Sergiy Kukunin <sergey.kuku...@gmail.com>
>> wrote:
>>
>>> Actually, that what I understood only in my last message - I can
>>> implement it right now. I'm pretty new to Elixir, so that wasn't obvious to
>>> me.
>>>
>>> Currently, it seems it's resolved, there are only suggestions to improve
>>> syntax, that are too minor.
>>>
>>> Thank everyone for assistance
>>>
>>> On Wed, Nov 7, 2018 at 9:15 PM Louis Pilfold <lo...@lpil.uk> wrote:
>>>
>>>> Hi Sergiy
>>>>
>>>> I'm afraid I don't follow. From what I understand of your proposal the
>>>> current defguard system meets your needs- what are you looking to add?
>>>>
>>>> Cheers,
>>>> Louis
>>>>
>>>> On Wed, 7 Nov 2018 at 18:38 Sergiy Kukunin <sergey.kuku...@gmail.com>
>>>> wrote:
>>>>
>>>>> 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 <sergey....@gmail.com>
>>>>>> 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, <ivan.y...@gmail.com> 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.
>>>>>>>>>>
>>>>>>>>> --
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