let is great and very important to have, but the syntax is very unusual. Before running them, try to guess what these do: function f1() a = 1 b = 2 c = let a = 3 b = 4 a+b end (a,b,c) end function f2() a = 1 b = 2 c = let a = 3 b …

Ok, so the cleanest way I can see to get the behavior I was expecting is julia> function f4() a = 1 b = 2 c = let local a = 3 local b = 4 a+b end (a,b,c) end f4 (generic function with 1 method…

[image] cscherrer: Well this just get weirder and weirder. Especially since you get [image] Mason: > julia> let let a = 1 end a end 1 > ``` But I get (Julia 1.5) julia> let let a = 1 end …

[image] StefanKarpinski: I don’t see it as that weird. It’s using the fact that let creates a local scope to hide a bunch of variables that are closed over. It is not using the ability of let to shadow an existing local binding with a new local binding. In this case there’s little difference sin…

[image] cscherrer: Is it correct to say that the first line of a let has hard local scope, and the rest has soft local scope? Hmm, no this still isn’t right. From the manual: When x = occurs in a local scope, Julia applies the following rules to decide what the expression means based on wh…

[image] lmiq: Yet one could perhaps expect that let block to result in undefined a anyway, since its scope is local (but since we are not modifying a it is considered as the global a ). Again, if we are talking about code outside the REPL, then let and for have no differences in this aspect, …

[image] Henrique_Becker: to not work because a is not defined in the for would be unnerving. Yes, of course, it was only that in those artificial examples one might be tricked.

The let construct is traditional, but it does two technically unrelated things which would perhaps be better disentangled: Create a local scope Create new bindings for a set of names in that scope We can imagine a scope ... end construct that just does the first. Then you could translate let a =…

You may already be aware of this, but the easier way to write function f() a = 1 b = 2 c = let local a = 3 local b = 4 a+b end (a, b, c) end is like this: function f() a = 1 b = 2 c = let a = 3, b = 4 a + b end (a, b, c) end I don’t know if it’s been made clear yet or not, but …

[image] CameronBieganek: You may already be aware of this, but the easier way to write function f() a = 1 b = 2 c = let local a = 3 local b = 4 a+b end (a, b, c) end is like this: function f() a = 1 b = 2 c = let a = 3, b = 4 a + b end (a, b, c) end Thanks, yes I’m aware …

[image] cscherrer: The point was that if we have many bindings or much longer ones, this gets very awkward. Just stick to one letter variable names. :wink:

`let` syntax weirdness

Internals & Design

cce January 3, 2021, 4:21pm 37

I think this would be an excellent syntax addition. It’s also not something you can implement with macros without introducing type instability, see this discussion thread.

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`let` syntax weirdness

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