4 ms·

I wish they didn't tbh, it makes me sad. Not allowing rebinding does provide a little bit of encouragement towards factoring things better, but I have seen man

• weatherlight 4y ago
  its a bit of a smell, but good elixir code doesn't have to worry about rebinding for bindings in the same scope. Jose Valim goes on as to why he made that design decision from the following: https://groups.google.com/g/elixir-lang-talk/c/w83lKZs4YS8/m/rFeb3Env-6YJ https://groups.google.com/g/elixir-lang-talk/c/w83lKZs4YS8/m... I will post the same answer from before: I like the explicitness of pattern matching. In Elixir, as soon as I see ^foo, I know I am matching. If there is no ^, I know the previous value regardless if it there is one or not, will be discarded. If pattern matching is not explicit, I always need to know if a variable was previously defined or not to know what is going to happen. To me this behaviour is non negotiable. In my experience, it is more likely to run into accidental matches than into accidental rebindings. Another possible limitation to the suggestion above can be related to macros. Let's suppose you have a macro that stores a value in a hygienic variable: defmacro do_something(a, b) do quote do var!(hello, Hygienic) = calculate_something(unquote(a), unquote(b)) end end Someone may call this macro multiple times but we always care about the last value of hello. If we make a distinction in between being assigned once and then multiple times, the macro simply won't work. Or the macro developer would need to work around this behaviour by inspecting the environment or we would need to provide some sort of functionality that does it for you. Maybe this behaviour could be built-in in var! or maybe we'd need to introduce something like: defmacro do_something(a, b) do quote do set!(hello, Hygienic) = calculate_something(unquote(a), unquote(b)) end end We can see how this is getting complex: 1. Use = to define variables 2. Use ^ for pattern matching 3. Use := for rebinding 4. Use set! in macros when you don't care if a variable was previously defined or not It is also interesting to point out that the := operator won't work for rebinding inside clauses. For example, how would you make the rebinding below explicit? x = true case false do x -> :ok end Of course, there are interesting consequences for making a distinction in between defining and rebinding a variable by introducing something like the := operator. We could have better control of the scope to provide better warnings or even make it easier to implement imperative for loops: x = 0 for i <- 1..5 do x := x + i end x #=> 15 But the fact Elixir have different ways for variables to be introduced in the scope, adding more rules can make the overall system very complex.