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Module Hashtbl = Core_hashtbl

Signature

For many students of ocaml, using hashtables is complicated by the functors. Here are a few tips:

For a list of hashtable functions see Hashtbl_intf.S.

To create a hashtable with string keys use String.Table. 


    let table = String.Table.create () ~size:4 in
    List.iter ~f:(fun (key, data) -> Hashtbl.set table ~key ~data)
      [ ("A", 1); ("B", 2); ("C", 3); ];
    Hashtbl.find table "C"

Here 4 need only be a guess at the hashtable's future size. There are other similar pre-made hashtables, eg Int63.Table or Host_and_port.Table.

To create a hashtable with a custom key type use Hashable. 


        module Key = struct
          module T = struct
            type t = String.t * Int63.t [@@deriving sexp]
            let compare = compare
            let hash = Hashtbl.hash
          end
          include T
          include Hashable.Make (T)
        end
        let table = Key.Table.create () ~size:4 in
        List.iter ~f:(fun (key, data) -> Hashtbl.set table ~key ~data)
          [ (("pi", Int63.zero), 3.14159);
            (("e", Int63.minus_one), 2.71828);
            (("Euler", Int63.one), 0.577215);
          ];
        Hashtbl.find table ("pi", Int63.zero)

Performance may improve if you define equal and hash explicitly, eg: 


        let equal (x, y) (x', y') = String.(=) x x' && Int63.(=) y y'
        let hash (x, y) = String.hash x + Int63.hash y * 65599
include Core_hashtbl_intf.Hashtbl
val hash : 'a -> int
val hash_param : int -> int -> 'a -> int
type ('a, 'b) t

We use [@@deriving sexp_of] but not [@@deriving sexp] because we want people to be explicit about the hash and comparison functions used when creating hashtables. One can use Hashtbl.Poly.t, which does have [@@deriving sexp], to use polymorphic comparison and hashing.

val sexp_of_t : ('a -> Sexplib.Sexp.t) -> ('b -> Sexplib.Sexp.t) -> ('a, 'b) t -> Sexplib.Sexp.t
include Invariant.S2 with type ('a, 'b) t := ('a, 'b) t
type ('a, 'b) t
val invariant : 'a Invariant_intf.inv -> 'b Invariant_intf.inv -> ('a, 'b) t Invariant_intf.inv
include Core_hashtbl_intf.Creators with type ('a, 'b) t := ('a, 'b) t with type 'a key = 'a with type ('a, 'b, 'z) create_options := ('a, 'b, 'z) Core_hashtbl_intf.create_options_with_hashable
type ('a, 'b) t
type 'a key = 'a
type ('key, 'data, 'z) create_options
val create : ('a key, 'b, unit -> ('a, 'b) t) create_options
val of_alist : ('a key, 'b, ('a key * 'b) list -> [
| `Ok of ('a, 'b) t
| `Duplicate_key of 'a key
]) create_options
val of_alist_report_all_dups : ('a key, 'b, ('a key * 'b) list -> [
| `Ok of ('a, 'b) t
| `Duplicate_keys of 'a key list
]) create_options
val of_alist_or_error : ('a key, 'b, ('a key * 'b) list -> ('a, 'b) t Or_error.t) create_options
val of_alist_exn : ('a key, 'b, ('a key * 'b) list -> ('a, 'b) t) create_options
val of_alist_multi : ('a key, 'b list, ('a key * 'b) list -> ('a, 'b list) t) create_options
val create_mapped : ('a key, 'b, get_key:('r -> 'a key) -> get_data:('r -> 'b) -> 'r list -> [
| `Ok of ('a, 'b) t
| `Duplicate_keys of 'a key list
]) create_options 
 create_mapped get_key get_data [x1,...,xn]
         = of_alist [get_key x1, get_data x1; ...; get_key xn, get_data xn]
val create_with_key : ('a key, 'r, get_key:('r -> 'a key) -> 'r list -> [
| `Ok of ('a, 'r) t
| `Duplicate_keys of 'a key list
]) create_options 
 create_with_key ~get_key [x1,...,xn]
         = of_alist [get_key x1, x1; ...; get_key xn, xn]
val create_with_key_or_error : ('a key, 'r, get_key:('r -> 'a key) -> 'r list -> ('a, 'r) t Or_error.t) create_options
val create_with_key_exn : ('a key, 'r, get_key:('r -> 'a key) -> 'r list -> ('a, 'r) t) create_options
val group : ('a key, 'b, get_key:('r -> 'a key) -> get_data:('r -> 'b) -> combine:('b -> 'b -> 'b) -> 'r list -> ('a, 'b) t) create_options
include Core_hashtbl_intf.Accessors with type ('a, 'b) t := ('a, 'b) t with type 'a key := 'a key with type ('a, 'z) map_options := ('a, 'z) Core_hashtbl_intf.no_map_options
type ('a, 'b) t
type 'a key
type ('a, 'z) map_options
val sexp_of_key : ('a, _) t -> 'a key -> Sexplib.Sexp.t
val clear : (_, _) t -> unit
val copy : ('a, 'b) t -> ('a, 'b) t
val fold : ('a, 'b) t -> init:'c -> f:(key:'a key -> data:'b -> 'c -> 'c) -> 'c
val iter_vals : (_, 'b) t -> f:('b -> unit) -> unit
val iteri : ('a, 'b) t -> f:(key:'a key -> data:'b -> unit) -> unit
val iter_keys : ('a, _) t -> f:('a key -> unit) -> unit
val iter : ('a, 'b) t -> f:(key:'a key -> data:'b -> unit) -> unit
val existsi : ('a, 'b) t -> f:(key:'a key -> data:'b -> bool) -> bool
val exists : (_, 'b) t -> f:('b -> bool) -> bool
val for_alli : ('a, 'b) t -> f:(key:'a key -> data:'b -> bool) -> bool
val for_all : (_, 'b) t -> f:('b -> bool) -> bool
val counti : ('a, 'b) t -> f:(key:'a key -> data:'b -> bool) -> int
val count : (_, 'b) t -> f:('b -> bool) -> int
val length : (_, _) t -> int
val is_empty : (_, _) t -> bool
val mem : ('a, _) t -> 'a key -> bool
val remove : ('a, _) t -> 'a key -> unit
val replace : ('a, 'b) t -> key:'a key -> data:'b -> unit
val set : ('a, 'b) t -> key:'a key -> data:'b -> unit
val add : ('a, 'b) t -> key:'a key -> data:'b -> [
| `Ok
| `Duplicate
]
val add_or_error : ('a, 'b) t -> key:'a key -> data:'b -> unit Or_error.t
val add_exn : ('a, 'b) t -> key:'a key -> data:'b -> unit
val change : ('a, 'b) t -> 'a key -> f:('b option -> 'b option) -> unit

change t key ~f changes t's value for key to be f (find t key).

val update : ('a, 'b) t -> 'a key -> f:('b option -> 'b) -> unit

update t key ~f is change t key ~f:(fun o -> Some (f o)).

val add_multi : ('a, 'b list) t -> key:'a key -> data:'b -> unit

add_multi t ~key ~data if key is present in the table then cons data on the list, otherwise add key with a single element list.

val remove_multi : ('a, _ list) t -> 'a key -> unit

remove_multi t key updates the table, removing the head of the list bound to key. If the list has only one element (or is empty) then the binding is removed.

val map : ('c, ('a, 'b) t -> f:('b -> 'c) -> ('a, 'c) t) map_options

map t f returns new table with bound values replaced by f applied to the bound values

val mapi : ('c, ('a, 'b) t -> f:(key:'a key -> data:'b -> 'c) -> ('a, 'c) t) map_options

like map, but function takes both key and data as arguments

val filter_map : ('c, ('a, 'b) t -> f:('b -> 'c option) -> ('a, 'c) t) map_options

returns new map with bound values filtered by f applied to the bound values

val filter_mapi : ('c, ('a, 'b) t -> f:(key:'a key -> data:'b -> 'c option) -> ('a, 'c) t) map_options

like filter_map, but function takes both key and data as arguments

val filter : ('a, 'b) t -> f:('b -> bool) -> ('a, 'b) t
val filteri : ('a, 'b) t -> f:(key:'a key -> data:'b -> bool) -> ('a, 'b) t
val partition_map : ('c, ('d, ('a, 'b) t -> f:('b -> [
| `Fst of 'c
| `Snd of 'd
]) -> ('a, 'c) t * ('a, 'd) t) map_options) map_options

returns new maps with bound values partitioned by f applied to the bound values

val partition_mapi : ('c, ('d, ('a, 'b) t -> f:(key:'a key -> data:'b -> [
| `Fst of 'c
| `Snd of 'd
]) -> ('a, 'c) t * ('a, 'd) t) map_options) map_options

like partition_map, but function takes both key and data as arguments

val partition_tf : ('a, 'b) t -> f:('b -> bool) -> ('a, 'b) t * ('a, 'b) t
val partitioni_tf : ('a, 'b) t -> f:(key:'a key -> data:'b -> bool) -> ('a, 'b) t * ('a, 'b) t
val find_or_add : ('a, 'b) t -> 'a key -> default:(unit -> 'b) -> 'b

find_or_add t k ~default returns the data associated with key k if it is in the table t, otherwise it lets d = default() and adds it to the table.

val find : ('a, 'b) t -> 'a key -> 'b option

find t k returns Some (the current binding) of k in t, or None if no such binding exists

val find_exn : ('a, 'b) t -> 'a key -> 'b

find_exn t k returns the current binding of k in t, or raises Not_found if no such binding exists.

val find_and_call : ('a, 'b) t -> 'a key -> if_found:('b -> 'c) -> if_not_found:('a key -> 'c) -> 'c

find_and_call t k ~if_found ~if_not_found

is equivalent to:

match find t k with Some v -> if_found v | None -> if_not_found k

except that it doesn't allocate the option.

val find_and_remove : ('a, 'b) t -> 'a key -> 'b option

find_and_remove t k returns Some (the current binding) of k in t and removes it, or None is no such binding exists

val merge : ('c, ('k, 'a) t -> ('k, 'b) t -> f:(key:'k key -> [
| `Left of 'a
| `Right of 'b
| `Both of 'a * 'b
] -> 'c option) -> ('k, 'c) t) map_options

Merge two hashtables.

The result of merge f h1 h2 has as keys the set of all k in the union of the sets of keys of h1 and h2 for which d(k) is not None, where:

d(k) =

  • f ~key:k (Some d1) None if k in h1 is to d1, and h2 does not map k;
  • f ~key:k None (Some d2) if k in h2 is to d2, and h1 does not map k;
  • f ~key:k (Some d1) (Some d2) otherwise, where k in h1 is to d1 and k in h2 is to d2.

Each key k is mapped to a single piece of data x, where d(k) = Some x.

val merge_into : f:(key:'a key -> 'b -> 'c option -> 'c option) -> src:('a, 'b) t -> dst:('a, 'c) t -> unit

Merge one hashtable into another.

After merge_into f src dst, for every key in src, key will be re-mapped in dst to v if f ~key d1 (find dst key) = Some v.

val keys : ('a, _) t -> 'a key list

Returns the list of all keys for given hashtable.

Returns the list of all data for given hashtable.

val data : (_, 'b) t -> 'b list

Returns the list of all data for given hashtable.

val filter_inplace : (_, 'b) t -> f:('b -> bool) -> unit

filter_inplace t ~f removes all the elements from t that don't satisfy f.

val filteri_inplace : ('a, 'b) t -> f:(key:'a key -> data:'b -> bool) -> unit
val filter_keys_inplace : ('a, _) t -> f:('a key -> bool) -> unit

replace_all t ~f applies f to all elements in t, transforming them in place

val replace_all : (_, 'b) t -> f:('b -> 'b) -> unit

replace_all t ~f applies f to all elements in t, transforming them in place

val replace_alli : ('a, 'b) t -> f:(key:'a key -> data:'b -> 'b) -> unit

filter_replace_all combines the effects of replace_all and filter_inplace

val filter_replace_all : (_, 'b) t -> f:('b -> 'b option) -> unit

filter_replace_all combines the effects of replace_all and filter_inplace

val filter_replace_alli : ('a, 'b) t -> f:(key:'a key -> data:'b -> 'b option) -> unit
val equal : ('a, 'b) t -> ('a, 'b) t -> ('b -> 'b -> bool) -> bool

equal t1 t2 f and similar t1 t2 f both return true iff t1 and t2 have the same keys and for all keys k, f (find_exn t1 k) (find_exn t2 k). equal and similar only differ in their types.

val similar : ('a, 'b1) t -> ('a, 'b2) t -> ('b1 -> 'b2 -> bool) -> bool
val to_alist : ('a, 'b) t -> ('a key * 'b) list

Returns the list of all (key,data) pairs for given hashtable.

val validate : name:('a key -> string) -> 'b Validate.check -> ('a, 'b) t Validate.check
val incr : ?by:int -> ('a, int) t -> 'a key -> unit
val hashable : ('key, _) t -> 'key Hashable.t
module Poly : sig .. end
module type Key = Core_hashtbl_intf.Key
module type S = Core_hashtbl_intf.S with type ('a, 'b) hashtbl = ('a, 'b) t
module type S_binable = Core_hashtbl_intf.S_binable with type ('a, 'b) hashtbl = ('a, 'b) t
module Make (Key : Key) : S with type key = Key.t
module Make_binable (Key : Key_binable) : S_binable with type key = Key.t