Type Checking with Witchcraft?

[expede]

[Ported from the old Witchcrafters Discourse]

@chouzar:

I just started playing with Algae and derived some very simple examples from the documentation:

defmodule Stats do
  import Algae

  defdata do
    hit_points :: non_neg_integer()
    experience :: non_neg_integer()
  end
end

defmodule Character do
  import Algae

  defdata do
    name :: String.t()
    stats :: Stats.t()
  end
end

Which by itself already creates the respective elixir structs and constructors:

iex(1)> %Character{}
%Character{name: "", stats: %Stats{experience: 0, hit_points: 0}}

iex(2)> Character.new "Player 1"
%Character{name: "Player 1", stats: %Stats{experience: 0, hit_points: 0}}

Also, the constructor typespecs:

iex(1)> h Character.new          

  @spec new(String.t(), Stats.t()) :: t()

Positional constructor, with args in the same order as they were defined in

Which is already super nice 😃.

But I was wondering if by any means Witchcraft may help at checking structs at runtime/complie time?

For example, with Algae it is possible to use constructors as:

iex(1)> Stats.new "four two", experience: "two four"
%Stats{experience: [experience: "two four"], hit_points: "four two"}

Even though both properties are specified as non_neg_integer().

@QuinnWilton pointed me to the TypeClass helpers as means of doing some checks at compile time.

Would it be possible or make sense to create a TypeClass that validated that those struct porperties should be integer values?

[details="Spoiler"]
My current guess is that it would be overkill/misuse of type classes since these are for checking the "semantics" of a module via property checking and not necessarily data checking.
[/details]

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@creminology:

I haven't used TypeClass yet, but have been meaning to try it.

Last year, I did successfully got Witchcraft working with Norm for runtime checks:
elixir-toniq/norm#30 (comment)

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@chouzar:

Thank you so much for sharing @creminology 😁

I think my initial assessment for TypeClass library was a bit sidetracked… I think TypeClass is more interested in validating that your protocols conform to a set of invariats via property tests; In that sense it has to do more with “behaviour” than “type-checks”.

Your examples are very helpful since implementing a TypeClass requires data generation, for which Norm is of great help.

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@expede:

Which is already super nice 😃.

🙌

But I was wondering if by any means Witchcraft may help at checking structs at runtime/complie time?

The short answer is no. Elixir does not have great support for type checking today, though you can get some limited checking with typespecs + Dialyzer (including when using the Algae types). Those tools all run as a separate step, and won't stop you from writing ill-typed programs unless you run them. There is some effort going towards adding a more robust type layer to BEAM languages, but nothing is available today.

Would it be possible or make sense to create a TypeClass that validated that those struct porperties should be integer values?

Typeclass isn't meant for type checking — it's more like suped-up protocols. You could absolutely write precondition checking in a constructor function ("smart constructor", which is actually more expressive than most type systems), but that will come with runtime costs in the general case.