Expect

Expect is a Julia implementation of the proposed C++ feature std::expected. The talk by Andrei Alexandrescu at CppCon 2018 gives a good summary of the reasons and is recommended watching. std::expected has some similarities to the Result type in Rust. It is still unknown to the author if this pattern is useful in a dynamically typed language like Julia, hence this package to try it out.

In short, std::expected (in this package exported as Expected) is a wrapper around a value (the good case) or an explanation for why creating that value failed (the bad case). The wrapped value is accessed using v[] which will in the bad case throw. Functions using Expected should annotate the return type of the function as ::Expected{T, E} where T is the type of the expected value and E is the type of the "unexpected" value (what we want to return in the bad case). If T == E, in the bad case, the return value v needs to be wrapped using unexpected(v) to distinguish it from the good case. Conversion based on the function return type annotation takes care of the rest. The function isassigned can be used to see if an Expected is in the good case.

Let's say we have some integer division routine that we want to return an error when we divide by zero (note that Julia already throws an error for this). We would use Expect in the following way:

using Expect
struct DivideByZeroException <: Exception end

function safe_divide(a::T, b::T)::Expected{T, DivideByZeroException} where {T <: Integer}
    b == 0 && return DivideByZeroException()
    return div(a, b)
end

Here is how a caller would use safe_divide in the good case:

julia> v = safe_divide(5, 2)
Expected{Int64,DivideByZeroException}(2, true)

julia> isassigned(v)
true

julia> v[]
2

And in the bad case:

julia> v = safe_divide(5, 0)
Expected{Int64,DivideByZeroException}(DivideByZeroException(), false)

julia> isassigned(v)
false

julia> v[]
ERROR: DivideByZeroException()
...

The advantage over just throwing in safe_divide is that this gives the caller a chance to more succintly deal with the error locally (without using try catch).