Why does Scala have path-dependent types?

Question

I've been doing some research on path-dependent types. The best description I could find for it was:

If L is a type label, then x.L and y.L are the same type iff x and y can be shown to refer to the same object.

This sometimes isn't the subtyping behaviour one would expect. I would expect that if L in the above example was indeed identical then that would be enough to make x.L and y.L indentical.

Is there any particular reason why Scala was designed this way?

score 9 · Accepted Answer · edited Sep 23 '19 at 04:33

9

The Scalable Component Abstractions paper has a good explanation on path dependent types and also a good example in Section 3: "Case study: subject/observer".

edited Sep 23 '19 at 04:33

smac89

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answered Apr 07 '11 at 15:02

michid

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score 4 · Answer 2 · edited Sep 23 '19 at 04:32

4

This paper explains it nicely. Basically, they're used to support abstract data type based programming and modularization.

edited Sep 23 '19 at 04:32

smac89

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answered Apr 07 '11 at 14:08

axel22

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CMIIW, but this paper seems to have absolutely nothing to do with path-dependent types. – Trevor Robinson Oct 07 '11 at 05:24
True - this should have linked to the Scalable Component Abstractions paper. – axel22 Oct 10 '11 at 12:28

score -8 · Answer 3 · answered Apr 07 '11 at 13:32

-8

Think about L as about type argument of generic class. Scala boasts about its type members but underlying JVM still has the same limitations.

answered Apr 07 '11 at 13:32

Basilevs

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Why does Scala have path-dependent types?

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