for x in … introduces a new x regardless of whether x is already defined:
The text is inaccurate but not entirely false.
julia> let
x = 2
for outer x = 1:5
end
@show x
end
x = 5
The keyword outer is necessary to use an already defined x, however, it cannot be in global scope (i.e., the variable must be from local scope).
This is a beautiful abomination 
It took me a little while to figure out why this even parses or what it means. Here’s what’s going on:
T where T is just a confusing way of writing Anywhere is a contextual keyword, so the T may be replace with where to get where where wherex isa Any for all values xisa isa Anyisa isa where where where as another name for true.Huh, never would have guessed:
julia> where = "I can assign a value to `where`."
"I can assign a value to `where`."
Looks like all syntax highlighters need to update their rules 
In general the Julia parser tries not to steal identifiers as keywords so there’s several words (abstract, as, doc, mutable, outer, primitive, type, var, isa, where) which are normally identifiers but have special parsing rules in some circumstances.
There’s two circumstance (quoting with : and field access with .) where words which are normally keywords become identifiers:
x.end = :for
A downside of this is that a syntax highlighter would need to be a full parser to get the highlighting 100% correct.
Unless you overload getproperty, there doesn’t appear to be many situations where you can actually get a property named end:
julia> struct A
end
julia> (a=1, end=2)
ERROR: syntax: unexpected "end"
In the first example, the REPL finished the struct definition before I could add another end.
VS Code does fine with highlighting symbols:
julia> struct A
var"end"
end
julia> A(1).end
1
Ha, ok. In that case I guess I’m obligated to report that VS Code does not get the highlighting for end correct:
Yeah the rules are (mostly?) local enough that syntax highlighting should be able to get them right though it’ll need some lookahead / lookbehind for things like outer which are somewhat more annoying. outer is a keyword here: for outer i = 1:10 and here for outer i (in) 1:10 but not here: for outer = 1:10 or here for outer in 1:10.
Speaking of outer… here’s a puzzle I’ve concocted using some other tricks up-thread, and another fairly serious WAT I recently discovered.
Does anything get printed here?
let isa = isa isa where where where
outer = false
for outer outer in isa
end
try
if outer isa Any
@info "Certainly true, right?"
end
finally
outer = false
catch
if outer
@info "Surprise?"
end
end
end
(Edit: I think it’s fair to say this isn’t exactly a WAT in itself, as it’s intentionally obtuse 
)
Neither corner case nor rare specimen.
println("What a lovely day!")
for i in 1 : 100_000_000
for j in 1 : 10_000
x = cos(j)
end
end
# Next day...
println("WAT!")); # Syntax error discovered
For me the actual WAT is writing code like this 
Still fun to see of course.
Even more baffling is reading this post, trying to figure out what’s going on and getting this help message.
_
_ _ _(_)_ | Documentation: https://docs.julialang.org
(_) | (_) (_) |
_ _ _| |_ __ _ | Type "?" for help, "]?" for Pkg help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 1.7.2 (2022-02-06)
_/ |\__'_|_|_|\__'_| | Official https://julialang.org/ release
|__/ |
help?> outer
search: count_zeros RoundToZero OutOfMemoryError ConcurrencyViolationError countlines count_ones RoundNearestTiesUp RoundNearestTiesAway
Couldn't find outer
Perhaps you meant one, ones, open, operm, pointer, quote, Timer, filter, put!, step, uperm, Number, Ptr or count
No documentation found.
Binding outer does not exist.
I know it is not a reserved keyword, but still…
Keywords with exclamation mark
julia> foo!(x; f!) = f!(x)
foo (generic function with 1 method)
julia> bar!(x) = begin x[1] = 0; return x end
bar! (generic function with 1 method)
julia> foo!([1, 2, 3]; f!=bar!)
ERROR: syntax: invalid keyword argument syntax "(f != bar!)" around REPL[18]:1
Stacktrace:
[1] top-level scope
@ REPL[18]:1
julia> foo!([1, 2, 3]; f! =bar!)
3-element Vector{Int64}:
0
2
3
Sorry for being a bit square—but this is really not a bad behavior.
f!=bar! example is pretending that spaces dont matter. they absolutely do, and that is well-knownIf we mark this sort of example as a “most harrowing WAT” then what do we call this
julia> f(x) = x
+ 1
1
julia> f(1)
1
Is this now an “unmitigated-disaster-level WAT” because there is no exception?
I hope we can agree that my example here is just a plain old bug because of bad code—and so is the f!=bar! example.
julia> x = [1, 2];
julia> d = Dict(x => 1)
Dict{Vector{Int64}, Int64} with 1 entry:
[1, 2] => 1
julia> push!(x, 3);
julia> d
Dict{Vector{Int64}, Int64} with 1 entry:
[1, 2, 3] => 1
julia> d[[1, 2]]
ERROR: KeyError: key [1, 2] not found
julia> d[[1, 2, 3]]
ERROR: KeyError: key [1, 2, 3] not found
Moral: Don’t use mutable keys.
Somehow, I am glad this doesn’t work!
The default behavior of == when comparing structs has always been a bit of a WAT to me.
julia> struct A
x::Vector{Int}
end
julia> A([1, 2]) == A([1, 2])
false
julia> mutable struct B
x::Int
end
julia> B(1) == B(1)
false
In my opinion, the default behavior should be to compare the values of the contents of the structs, regardless of whether the structs or the contents of the structs are mutable.
That’s actually (kind-of) already tracked as a 2.0 issue, though there’s probably more discussion to be had on how exactly this should work.
This is not so much of a WAT for everyone, but… WAT!!!
julia> macro weird(a::Int)
@show a, typeof(a)
1
end
@weird (macro with 1 methods)
julia> macro weird(a::Float64)
@show a, typeof(a)
2a
end
@weird (macro with 2 methods)
julia> macro weird(a::String)
@show a, typeof(a)
a^3
end
@weird (macro with 3 methods)
julia> macro weird(a::Symbol)
@show a, typeof(a)
:($(a)^4)
end
@weird (macro with 4 methods)
julia> macro weird(a::Expr)
@show a, typeof(a)
a
end
@weird (macro with 5 methods)
julia> @weird 10
(a, typeof(a)) = (10, Int64)
1
julia> @weird 1.0
(a, typeof(a)) = (1.0, Float64)
2.0
julia> @weird "Hi"
(a, typeof(a)) = ("Hi", String)
"HiHiHi"
julia> @weird im
(a, typeof(a)) = (:im, Symbol)
1 + 0im
julia> @weird "Hi" ^ 3
(a, typeof(a)) = (:("Hi" ^ 3), Expr)
"HiHiHi"
I would imagine that macro calling logic would consider all arguments as “typeless”, but that doesn’t seem to be the case.
(Without knowing the specifics, I guess that macros are “just” functions underneath that have a special syntax and run on a different moment under the Julia parsing-lowering-compilation-execution pipeline. i.e., just after the parsing and before the lowering).