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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