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by antisemiotic 2555 days ago
In Haskell this can be pushed further, by making existential types, i.e. opaque type that's only known to implement some type class. This doesn't seem to be that useful in Haskell (I think it's because of the immutability), but trait objects in Rust are pretty much the same and are used more often.

As a side note, I hope that the situation with pre- and postcondition checking improves (runtime verficiation like in Racket seems too heavyweight for me, dependent types alone are kind of clunky, type refinements are either bolted onto existing languages (LiquidHaskell, F7) and don't always fit, or exist in very obscure ones (F*)), and we can drop information hiding for the sake of safety alone. I don't like how today the "safe" option is to severly limit what you can do with a data type (want to safely index an array? just use an iterator! oh, you wanted to index two arrays at once? just zip them and hope the compiler makes the tuples go away! you wanted O(1) indexing? too bad, no can do).
1 comments
lmm 2555 days ago
> This doesn't seem to be that useful in Haskell (I think it's because of the immutability)

I suspect it's because of the polymorphism. Why write a function that operates on some unknown a for which a Foo implementation exists when it's so easy to write a function that operates on any a for which a Foo implementation exists?

The only time I've seen existentials used is for safety - in particular ST-style monads where a fancy type is used to ensure that you can't "leak" state out of the monadic context.

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antisemiotic 2555 days ago
It indeed makes no sense for passing arguments, but it does for storing things. For example, both `[Int]` and `[Float]` can be passed to a function of type `Num a => [a] -> a`, but these are distinct types, and neither can store a mix of ints and floats.

It ultimately boils down to the difference between static and dynamic polymorphism (Even more visible in Rust, where regular polymorphism works exactly like C++ templates, while trait objects pack a "v-table" together with a structure. In Haskell it's a little more blurry since "static" polymorphism is already implemented in a way that doesn't easily translate to templates, for example allowing polymorphic recursion).

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marcosdumay 2555 days ago
Optparse-applicative has a very nice use of existential types to break a definition loop.
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