Sketch of lowering for improved exception handling

exc_lowering.jl

module Asdf

struct ErrorResult{E}
    e::E
end

#=
function bar()
    throw ErrorException("Blah")
end

function foo(x::Int)
    if x > 1
        y = bar() throws Exception
    else
        y = 2
    end
    y + 2
end

function foo(s::String)
    "Hi "*s
end

x = 10
try
    z = foo(x) throws Exception
    @show z
catch exc::ErrorException
    @show exc
end

try
    z = foo(x)
    @show z
catch exc
    @show exc
end

foo("A")

=#

# Turns into:
function bar(_maybe_wrap_error_1)
    e = _maybe_wrap_error_1(ErrorException("Blah"))
    if e isa ErrorResult
        return e
    else
        throw(e)
    end
end

function _foo(_maybe_wrap_error_1, x::Int)
    if x > 1
        _maybe_wrap_error_2(e) = e isa Exception ? _maybe_wrap_error_1(e) : e
        y = bar(_maybe_wrap_error_2)
        if y isa ErrorResult
            return y
        end
    else
        y = 2
    end
    y + 2
end

@inline foo(x::Int) = _foo(identity, x)

function foo(s::String)
    "Hi "*s
end
# Don't really need the following definition, as cannot throw
# @inline _foo(_maybe_wrap_error_1, s::String) = foo(s)


x = 2
# catch
_maybe_wrap_error_0(e) = e isa ErrorException ? ErrorResult(e) : e
# try body
_maybe_wrap_error_1(e) = e isa Exception ? _maybe_wrap_error_0(e) : e
z_ = _foo(_maybe_wrap_error_1, x)
if !(z_ isa ErrorResult)
    z = z_
    @show z
else
    exc = z_.e
    # catch body
    @show exc
end

try
    z = foo(x)
    @show z
catch exc
    @show exc
end

@show foo("A")

end