Add at-try(x) macro

[kmsquire]

• Immediately returns an error x is an error, otherwise unwraps x

This mimics the behavior of the macro of the same name in Rust.

Rust has actually moved away from this macro, now adding ? to the end of Result types instead. That option isn't available to Julia (unless, perhaps, Julia moves towards using ResultTypes by default.)

I really wanted to call this @?, but question marks are not valid identifiers (they're syntax). I also tried @/ (a ? without pressing shift, on US keyboards at least). But that looked funny, so I went with precedence.

One other Rust-like behavior that this assumes is that exceptions higher in the stack will be converted to different exceptions as the stack unwinds. This is not common in Julia code (because exceptions are usually thrown, so there's little need to convert them). An additional Result convert method was added to make this easier, but users will still need to add convert methods for converting between their own exception types.

Example from the tests:

using ResultTypes

struct BarError <: Exception end
struct FooError <: Exception end
Base.convert(::Type{FooError}, ::BarError) = FooError()

function isbar(x)::Result{Bool, BarError}
    x == "foo" && return BarError()
    return x == "bar"
end

function foo(x)::Result{Int, FooError}
    bar = @try isbar(x)
    return bar ? 42 : 13
end

foobar = foo("bar") # unwrap(foobar) === 42
foofoo = foo("foo") # unwrap_error(foofoo) === FooError()
football = foo("tball") # unwrap(football) === 13

[codecov-io]

Codecov Report

Merging #17 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #17   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          29     38    +9     
=====================================
+ Hits           29     38    +9

Impacted Files	Coverage Δ
src/ResultTypes.jl	100% <100%> (ø)	:arrow_up:

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[iamed2]

Looks good, but here's an additional suggestion:

What about an optional second argument that is a replacement exception to return? This will prevent people from having to necessarily pirate exception conversions in some cases, while still adding the convenience of the compact early return syntax.

[kmsquire]

Okay, @iamed2, I added a version which takes an error as the second argument. Let me know what you think and if you want any changes.

[kmsquire]

I just realized that ! is the syntax for macro calls in rust (I've only dabbled in the language). Since we're not modifying our arguments here, I'm going to change this to @try.

[kmsquire]

... which of course doesn't work because try is a keyword... ( sigh )

[kmsquire]

Actually, I just learned that it is possible to define @try:

https://discourse.julialang.org/t/using-reserved-words-syntax-try-as-macro-names/34063

Will push that change again.

[kmsquire]

Ready for re-review.

[kmsquire]

Sorry, forgot that I was using an autoformatter. Will fix.

[kmsquire]

Should be good to go!

[iamed2]

Thanks!