Re: [Caml-list] a question about GADTs

[Jesper Louis Andersen <jesper.louis.andersen@gmail.com>]

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On Sun, Jun 17, 2018 at 8:39 AM Viet Le <vietlq85@gmail.com> wrote:

>
> I also came across a blog about writing GraphQL server using OCaml & GADT.
> Worth checking out.
>
>
The use there is pretty good. In GraphQL you have a query language in which
fields are bound to functions with arguments. Andreas (the GraphQL author)
takes the graphql schema and uses a GADT to derive the correct type for
said function. In short: the function will have the correct type and schema
changes will require updates on the necessary functions.

But you could argue this is still within the bounds of PL:

GraphQL is a language

Their spec doesn't show it, but it is best implemented as such: have a
statics phase and a dynamics phase, type check vigorously in the statics
phase. Most of what they call "validations" are just standard type checking
hygiene.
Also, the types are moded/polarized in that certain types flows from the
client to the server (which means you shouldn't trust the client) and other
types flows from the server to the client (which means you shouldn't trust
the server).

I have a hunch that there is a small core language within GraphQL best
realized as a lambda-calculus (with added primops corresponding to
execution of user-defined code). But I have yet to try implementing that.

---------

A use I've seen Janes St. doing is to use a GADT as a witness for a given
implementation. This allows the programmer to define a generic
implementation of, say 'a array, and then later specialize the bool array
variant into a bitset on words, or a more advanced data structure where the
bitset is a tree of some kind. The GADT then "selects" for the given
implementation in the code base.

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