66 lines
2 KiB
Markdown
66 lines
2 KiB
Markdown
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## Gmap - heterogenous maps over a GADT
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0.3.0
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Gmap exposes the functor `Make` which takes a key type (a
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[GADT](https://en.wikipedia.org/wiki/Generalized_algebraic_data_type) 'a key)
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and outputs a type-safe Map where each 'a key is associated with a 'a value.
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This removes the need for additional packing. It uses OCaml's stdlib
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[Map](http://caml.inria.fr/pub/docs/manual-ocaml/libref/Map.html) data
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structure.
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```OCaml
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type _ key =
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| I : int key
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| S : string key
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module K = struct
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type 'a t = 'a key
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let compare : type a b. a t -> b t -> (a, b) Gmap.Order.t = fun t t' ->
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let open Gmap.Order in
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match t, t' with
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| I, I -> Eq | I, _ -> Lt | _, I -> Gt
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| S, S -> Eq
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end
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module M = Gmap.Make(K)
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let () =
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let m = M.empty in
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...
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match M.find I m with
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| Some x -> Printf.printf "got %d\n" x
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| None -> Printf.printf "found nothing\n"
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```
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This is already an exhaustive pattern match: there is no need for another case
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(for the constructor `B`) since the type system knows that looking for `A` will
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result in an `int`.
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Motivation came from parsing of protocols which usually specify optional values
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and extensions via a tag-length-value (TLV) mechanism: for a given tag the
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structure of value is different - see for example IP options, TCP options, DNS
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resource records, TLS hello extensions, etc.
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Discussing this problem with Justus Matthiesen during summer 2017, we came up
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with this design. Its main difference to Daniel C. Bünzli's
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[hmap](http://erratique.ch/software/hmap) is that in gmap the key-value GADT
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type must be provided when instantiating the functor. In hmap, keys are created
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dynamically.
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## Documentation
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[](https://travis-ci.org/hannesm/gmap)
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[API documentation](https://hannesm.github.io/gmap/doc/) is available online.
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## Installation
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You need [opam](https://opam.ocaml.org) installed on your system. The command
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`opam install gmap`
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will install this library.
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