Elixir 功能在什么时候会成为误导？

Question

Generally speaking, Functional Programming prides itself for being clearer and concise. The fact that you don't have side-effects/state management makes it easier for developers to reason about their code and assure behaviours. How far does this truth reach?

I'm still learning Elixir but given the code from Coding Gnome:

def make_move(game = %{ game_state: state }, _guess)
  when state in [:won, :lost] do
    ...
end

def make_move(game = %{ game_state: state }, _guess)
  when state in [:pending] do
    ...
end

def make_move(game, guess) do
  ...
end

One could write it without any fanciness in Javascript as:

const makeMove = (game, guess) => {
  switch(game.state) {
    case 'won':
      return makeMoveFinalState();
    case 'lost':
      return makeMoveFinalState();
    case 'pending':
      return makeMovePending();
  }
}

Disregarding all the type/struct safety provided by Elixir compiler, an Elixir programmer would have to read the whole file before making sure that there wasn't a function with a different signature hijacking another, right? I feel that this increases the overhead while a programmer, because it's yet another thing you have to think about, even before looking at the implementation of the function.

Besides that, it feels to me as a misdirection because you can't be 100% sure that a case is ending up in that general make_move function unless you know beforehand all others and the signatures types, while with a conditional you have a clearer path of flow.

Could this be rewritten in a better way? At what point does these abstractions start to weight in the programmer?

Answer 1

I think this boils down mostly to preference and usually simple exercises with pattern matching with simple conditions do not show the range of "clarity" pattern matching can provide. But I'm suspect because I prefer pattern matching, any way, I'm gonna bite.

In this case, the switch could be said to be more readable and straightforward, but note that there's nothing preventing you from writing a very similar thing in Elixir (or erlang)

def make_move(game = %{ game_state: state }, _guess) do
    case state do
       state when state in [:won, :lost] -> # do things
       :pending -> # do things
       _else -> # do other things
    end
end

Regarding the placement of different function clauses for the same function name, elixir will emit a warning if they're not grouped together, so that ends up just being your responsibility to write them together and in the correct order (it will also warn you if any of the branches is by definition unreachable, like placing a catch all before any specific branch that has matchings).

But I think that if for instance you add a slight change of the matching requirements for the pending state, then in my view it starts becoming clearer to write it in the erlang/elixir way. Say that when the state is pending there are two different execution paths, depending if it's your turn or something else.

Now you could write 2 specific branches for that with just function signatures:

def make_move(game = %{ game_state: :pending, your_turn: true }, _guess) do
    # do stuff
end

def make_move(game = %{ game_state: :pending }, _guess) do
    # do stuff
end

To do that in JS you would need to have either another switch, or another if. If you have more complex matching patterns then it easily becomes harder to follow, while on elixir I think the paths are quite clear.

If the other conditions could be more thornier, say when it's :pending and there's nothing on a stack key that holds a list, then again matching that becomes:

def make_move(game = %{ game_state: :pending, your_turn: true, stack: [] }, _guess) do

Or if there's another branch where it depends if the first item in the stack was something specific:

def make_move(game = %{ game_state: :pending, your_turn: true, player_id: your_id, stack: [%AnAlmostTypedStruct{player: your_id} | _] }, _guess) do

Here erlang/elixir would only match this if your_id was the same in both places where it's used in the pattern.

And also, you say "without fanciness" in JS, but different function heads/arity/pattern matching is nothing fancy in Elixir/Erlang, it's just like the language has support for switch/case based statements at a much lower level (at the module compilation level?).

I for one would love to have effective pattern matching & different function clauses (not destructuring only) in JS.