# Julia - is a fractal of good design?

**URL:** <https://discourse.julialang.org/t/julia-is-a-fractal-of-good-design/8640>\
**Category:** Internals & Design\
**Tags:** proposal, development, design\
**Created:** [January 27, 2018, 4:57pm UTC](https://discourse.julialang.org/t/julia-is-a-fractal-of-good-design/8640 "2018-01-27T16:57:32Z")\
**Posts on this page:** 1\
**Showing post:** 4

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**Author:** ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)\
**Post date:** [January 28, 2018, 12:52am UTC](https://discourse.julialang.org/t/julia-is-a-fractal-of-good-design/8640/4 "2018-01-28T00:52:41Z")

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And split off into a second reply:

Covariance of tuple types is cool. The fact that dispatch always goes to a concrete type is cool.

Sometimes (rarely) people want something else, e.g. [https://discourse.julialang.org/t/missing-data-and-namedtuple-compatibility/8136/6](https://discourse.julialang.org/t/missing-data-and-namedtuple-compatibility/8136/6). Something that would be cool for this are invariant tuples. An `Ituple{Union{Int,Missing}, Union{Int,Missing}}`would be a concrete type to dispatch on; basically `Ituple{A,B}` would by syntactic sugar for `struct Itup_23456 v1::A v2::B end`. This fights the exponential explosion of needed specializations: Pass via Ituples; then your function becomes slower, because it needs all kind of type-dependent branches; but, on the other hand, dispatch into this function, and the amount of codegen becomes faster. As far as I understood the related discussion, named tuples can currently be abused for this. But I feel that this deserves to be a first class feature, and I feel like named tuples want to become covariant in the long run. I apologize if I grossly misunderstood the current state of things, or if I failed at RTFM (e.g. maybe `@nospecialize` can be be made to do all this, easily).

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