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.your...@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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