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81 lines
2.3 KiB
Markdown
81 lines
2.3 KiB
Markdown
# More about OCaml-containers
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This document contains more information on some modules of Containers.
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```ocaml
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# #require "containers";;
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```
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## Hash combinators: `CCHash`
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Although OCaml provides polymorphic hash tables (`('a,'b) Hashtbl.t`)
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using the polymorphic equality `(=)` and hash `Hashtbl.hash`, it is often
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safer and more efficient to use `Hashtbl.Make` (or the extended `CCHashtbl.Make`)
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with custom equality and hash functions.
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`CCHash` provides combinators for writing hash functions:
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```ocaml
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# module H = CCHash;;
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module H = CCHash
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# let hash1 : (int * bool) list H.t = H.(list (pair int bool));;
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val hash1 : (int * bool) list H.t = <fun>
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```
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```ocaml non-deterministic=output
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# hash1 [1, true; 2, false; 3, true];;
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- : int = 636041136
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# hash1 CCList.(1 -- 1000 |> map (fun i->i, i mod 2 = 0));;
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- : int = 845685523
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# hash1 CCList.(1 -- 1001 |> map (fun i->i, i mod 2 = 0));;
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- : int = 381026697
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```
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The polymorphic hash function is still present, as `CCHash.poly`.
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The functions `CCHash.list_comm` and `CCHash.array_comm` allow to hash
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lists and arrays while ignoring the order of elements: all permutations
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of the input will have the same hash.
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## Parser Combinator: `CCParse`
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The module `CCParse` defines basic parser combinators on strings.
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Adapting [angstrom's tutorial example](https://github.com/inhabitedtype/angstrom#usage)
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gives the following snippet.
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Note that backtracking is explicit in `CCParse`, hence
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the use of `try_` to allow it in some places.
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Explicit memoization with `memo` and `fix_memo` is also possible.
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```ocaml
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open CCParse.Infix
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module P = CCParse
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let parens p = P.try_ (P.char '(') *> p <* P.char ')'
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let add = P.char '+' *> P.return (+)
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let sub = P.char '-' *> P.return (-)
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let mul = P.char '*' *> P.return ( * )
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let div = P.char '/' *> P.return ( / )
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let integer =
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P.chars1_if (function '0'..'9'->true|_->false) >|= int_of_string
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let chainl1 e op =
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P.fix (fun r ->
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e >>= fun x -> P.try_ (op <*> P.return x <*> r) <|> P.return x)
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let expr : int P.t =
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P.fix (fun expr ->
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let factor = parens expr <|> integer in
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let term = chainl1 factor (mul <|> div) in
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chainl1 term (add <|> sub))
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```
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Now we can parse strings using `expr`:
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```ocaml
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# P.parse_string expr "4*1+2";; (* Ok 6 *)
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- : int P.or_error = Result.Ok 6
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# P.parse_string expr "4*(1+2)";; (* Ok 12 *)
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- : int P.or_error = Result.Ok 12
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```
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