Why ∀ is an invalid character?

Internals & Design
8

You need to use string macros here or a special repl mode if you want this syntax.

using ReplMaker
using REPL: LineEdit

iscomplete(x) = true
function iscomplete(ex::Expr)
    if ex.head == :incomplete
        false
    else
        true
    end
end

function valid_code(s)
    input = String(take!(copy(LineEdit.buffer(s))))
    iscomplete(parse_forall(input))
end

function parse_forall(s::String)
    Meta.parse(replace(s, "∀" => "for"))
end

julia> initrepl(parse_forall,
                prompt_text="∀ julia> ",
                prompt_color = :blue,
                start_key=')',
                mode_name="∀_mode",
                valid_input_checker=valid_code)
REPL mode ∀_mode initialized. Press ) to enter and backspace to exit.

∀ julia> [x^2 + 1 ∀ x ∈ -3:3]
7-element Array{Int64,1}:
 10
  5
  2
  1
  2
  5
 10

∀ julia> ∀ x ∈ 1:4
             @show x + 1
         end
x + 1 = 2
x + 1 = 3
x + 1 = 4
x + 1 = 5
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