Julia Result Patterns

A result pattern is a small value that holds either a good value or an error note. The caller checks the note instead of catching an exception.

A Result holds a value and an error note

The success case holds 42 with an empty note. The check on the note prints true.

struct Result
    value::Int
    error::String
end
ok = Result(42, "")
println(ok.value)
println(ok.error == "")

The error case fills the note

The failure case holds 0 with the note missing. The note itself is printed.

struct Result
    value::Int
    error::String
end
bad = Result(0, "missing")
println(bad.error)

An unwrap helper returns the value or a fallback

The helper returns 42 for the success case. It returns 7 for the failure case.

struct Result
    value::Int
    error::String
end
function unwrap(r)
    if r.error == ""
        return r.value
    end
    return 7
end
println(unwrap(Result(42, "")))
println(unwrap(Result(0, "missing")))

Example

struct Result
    value::Int
    error::String
end
function unwrap(r)
    if r.error == ""
        return r.value
    end
    return 7
end
function half_result(x)
    if x < 0
        return Result(0, "negative")
    end
    return Result(div(x, 2), "")
end
println(unwrap(half_result(20)))
println(unwrap(half_result(-4)))

Key points

Result: a small struct holding a value field and an error field.
Success: an empty error note means the value field holds the answer.
Failure: a filled error note means the value field holds no answer.
Unwrap: a helper that returns the value field or a fixed fallback.
Fallback: a fixed value used when the error note is filled.

In this example, half_result returns a Result holding 10 for 20, so unwrap prints 10. For -4 it returns a Result with the note negative, so unwrap prints the fallback 7.

Note

the caller checks the error note with a plain if, not with a catch.

Try it Yourself

Edit the code, then press Run.

Running is simulated · JuliaHub

struct Result
    value::Int
    error::String
end
function unwrap(r)
    if r.error == ""
        return r.value
    end
    return 7
end
function half_result(x)
    if x < 0
        return Result(0, "negative")
    end
    return Result(div(x, 2), "")
end
println(unwrap(half_result(20)))
println(unwrap(half_result(-4)))

Exercise

unwrap is called on Result(0, "missing"). The fallback is 7. What does unwrap return?

0 missing 42 7