# SE-0244: Opaque Result Types

**URL:** <https://forums.swift.org/t/se-0244-opaque-result-types/21252>\
**Category:** Proposal Reviews\
**Created:** [March 5, 2019, 9:40pm UTC](https://forums.swift.org/t/se-0244-opaque-result-types/21252 "2019-03-05T21:40:52Z")\
**Posts on this page:** 1\
**Showing post:** 70

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**Author:** ![fractal](https://avatars.discourse-cdn.com/v4/letter/f/2bfe46/32.png) [@fractal](https://forums.swift.org/u/fractal)\
**Post date:** [March 11, 2019, 10:22pm UTC](https://forums.swift.org/t/se-0244-opaque-result-types/21252/70 "2019-03-11T22:22:11Z")

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+1 for the concept, but -1 for the syntax because it doesn't allow explicit constraints, and doesn't scale to multiple opaque types. I quite liked the syntax proposed in the original discussion thread by @anthonylatsis (here: [Opaque result types - #88 by anthonylatsis](https://forums.swift.org/t/opaque-result-types/15645/88)) which explicitly named the opaque "thing" so it could be constrained in a `where` clause. To summarize what I think the linked post was getting at, it would look like:

```
func foo() -> some T
    where T: Collection, T.Element == Int
{ 
    return [1, 2, 3]
}

```

This has several advantages. For one, it doesn't require any new syntax for the constraints and instead re-uses the existing `where` clause that is already familiar. This allows it to scale to multiple types:

```
func foo() -> (some T, some Q)
    where T: Collection, T.Element == Int, Q: Numeric
{ 
    return ([1, 2, 3], 4)
}

```

And finally it allows opaque constraints to mix with generic constraints (composability!) which I think will be really important for the opaque type concept in general:

```
func foo<R>(input: R) -> some T
    where T: Collection, T.Element == R
{ 
    return [input]
}

```

I would of course support type inference to allow omission of some or all of the above constraints if they can be inferred from the return type:

```
func foo() -> some T: Collection
{ 
    // T.Element == Int is inferred
    return [1, 2, 3]
}

```

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