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Records, sums, cases, and exceptions: Row-polymorphism at work

Video: Records, sums, cases, and exceptions: Row-polymorphism at work, Matthias Blume.

I will present the design of a programming language (called MLPolyR) whose type system makes significant use of row polymorphism (Rémy, 1991). MLPolyR (Blume et al. 2006) is a dialect of ML and provides extensible records as well as their exact dual, polymorphic sums with extensible first-class cases.

Found this to be an enjoyable and thorough overview of MLPolyR, a language created for a PL course that goes all-out on various dimensions of row polymorphism, resulting in a small yet powerful language. (previously)

By Manuel J. Simoni at 2012-11-01 10:12 | Implementation | Type Theory | other blogs | 20070 reads

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Terminology?

Can you please explain in more detail what you mean when you say "polymorphic sums with extensible first-class cases"?

I can't make the phrase make any sense not already made by "dynamically typed variables in the presence of a runtime-extensible set of types" -- the sort of thing that could be implemented as a pointer to an allocated buffer plus a typetag, if the typetag's value is one element of a set which may acquire more members (for interactively or programmatically defined types) during runtime.

This is based mostly on the extant-if-somewhat-rare terminology for the types of records/objects as "products" of the types of their fields; I don't think I'd heard anyone use "sum" in an analogous way before. It's also based on what you said about them being dual to extensible records; If types can be dynamically defined, then it's trivial to define a typetag/case for an arbitrarily extended record -- which will be a new such "product" type -- and allocate a "sum" which is, at a given moment or in a given case, an instance of the new "product" type.

Am I guessing your meaning correctly here?

Ray Dillinger

By Ray Dillinger at Thu, 2012-11-01 16:48 | login or register to post comments

I can't make the phrase make

I can't make the phrase make any sense not already made by "dynamically typed variables in the presence of a runtime-extensible set of types"

Yes, they are indeed very similar, but row polymorphism gives you full type safety (e.g. accessing a record field that isn't there is a compile-time error) - all the while offering full type inference.

-- the sort of thing that could be implemented as a pointer to an allocated buffer plus a typetag, if the typetag's value is one element of a set which may acquire more members (for interactively or programmatically defined types) during runtime.

Yes, that is actually similar to how polymorphic variants are implemented in O'Caml. See page 4 in Programming with Polymorphic Variants.

By Manuel J. Simoni at Fri, 2012-11-02 12:22 | login or register to post comments

The second and third google

The second and third google result for "first class cases": Extensible Programming with First-Class Cases.

By naasking at Thu, 2012-11-01 17:47 | login or register to post comments

What, no lmgtfy link?

FYI, if you search for "polymorphic sums with extensible first-class cases", it's the first, second, and third links. :)

By Matt M at Thu, 2012-11-01 20:55 | login or register to post comments