After a 2 year Clojure stint I find it very hard to explain the clarity that comes with immutability for programmers used to trigger effects with a mutation.
I think it may be one of those things you have to see in order to understand.
replies(17):
I think it may be one of those things you have to see in order to understand.
With a very basic concrete example:
x = 7
x = x + 3
x = x / 2
Vs
x = 7
x1 = x + 3
x2 = x1 / 2
Reordering the first will have no error, but you'll get the wrong result. The second will produce an error if you try to reorder the statements.
Another way to look at it is that in the first example, the 3rd calculation doesn't have "x" as a dependency but rather "x in the state where addition has already been completed" (i.e. it's 3 different x's that all share the same name). Doing single assignment is just making this explicit.
As in - it's not very "constant" if you keep re-making it in your loop, right?
Whereas "immutable" throws away that extra context and means "whatever variable you have, for however long you have it, it's unchangeable."
you cant change a constant though
Immutability doesn’t have this connotation.
You’re allowed to rebind a var defined within a loop, it doesn’t mean that you can’t hang on to the old value if you need to.
With mutability, you actively can’t hang on to the old value, it’ll change under your feet.
Maybe it makes more sense if you think about it like tail recursion: you call a function and do some calculations, and then you call the same function again, but with new args.
This is allowed, and not the same as hammering a variable in place.
for (0..5) |i| {
i = i + 1;
std.debug.print("foo {}\n", .{i});
}
To potentially make it clearer that this is not mutation of a constant between iterations, technically &i could change between iterations, and the program would still be correct. This is not true with a c-style for loop using explicit mutation.