Module Stdlib.Hashtbl

Contents

Instructions: Use this module in your project

In the IDE (CLion, Visual Studio Code, Xcode, etc.) you use for your DkSDK project:

  1. Add the following to your project's dependencies/CMakeLists.txt:

    Copy
    DkSDKProject_DeclareAvailable(ocaml
    CONSTRAINT "= 4.14.0"
    FINDLIBS str unix runtime_events threads dynlink)
DkSDKProject_MakeAvailable(ocaml)

  2. Add the Findlib::ocaml library to any desired targets in src/*/CMakeLists.txt:

    Copy
    target_link_libraries(YourPackage_YourLibraryName
     # ... existing libraries, if any ...
     Findlib::ocaml)

  3. Click your IDE's Build button

Not using DkSDK?

FIRST, do one or all of the following:

  1. Run:

    Copy
    opam install ocaml.4.14.0

  2. Edit your dune-project and add:

    Copy
    (package
  (name YourExistingPackage)
  (depends
  ; ... existing dependenices ...
  (ocaml (>= 4.14.0))))

    Then run:

    Copy
    dune build *.opam # if this fails, run: dune build

  3. Edit your <package>.opam file and add:

    Copy
    depends: [
  # ... existing dependencies ...
  "ocaml" {>= "4.14.0"}
]

    Then run:

    Copy
    opam install . --deps-only

FINALLY, add the library to any desired (library)and/or (executable) targets in your **/dune files:

Copy
(library
  (name YourLibrary)
  ; ... existing library options ...
  (libraries
    ; ... existing libraries ...
    ))

(executable
  (name YourExecutable)
  ; ... existing executable options ...
  (libraries
    ; ... existing libraries ...
    ))

Generic interface

type ``(!'a, !'b) t

The type of hash tables from type 'a to type 'b.

valcreate : ``?random:bool->``int-> ``('a, 'b)`` t

Hashtbl.create n creates a new, empty hash table, with initial size n. For best results, n should be on the order of the expected number of elements that will be in the table. The table grows as needed, so n is just an initial guess.

The optional ~``random parameter (a boolean) controls whether the internal organization of the hash table is randomized at each execution of Hashtbl.create or deterministic over all executions.

A hash table that is created with ~``random set to false uses a fixed hash function (hash) to distribute keys among buckets. As a consequence, collisions between keys happen deterministically. In Web-facing applications or other security-sensitive applications, the deterministic collision patterns can be exploited by a malicious user to create a denial-of-service attack: the attacker sends input crafted to create many collisions in the table, slowing the application down.

A hash table that is created with ~``random set to true uses the seeded hash function seeded_hash with a seed that is randomly chosen at hash table creation time. In effect, the hash function used is randomly selected among 2^{30} different hash functions. All these hash functions have different collision patterns, rendering ineffective the denial-of-service attack described above. However, because of randomization, enumerating all elements of the hash table using fold or iter is no longer deterministic: elements are enumerated in different orders at different runs of the program.

If no ~``random parameter is given, hash tables are created in non-random mode by default. This default can be changed either programmatically by calling randomize or by setting the R flag in the OCAMLRUNPARAM environment variable.

  • before 4.00.0

    the ~``random parameter was not present and all hash tables were created in non-randomized mode.

val clear : ``('a, 'b)`` t -> unit

Empty a hash table. Use reset instead of clear to shrink the size of the bucket table to its initial size.

val reset : ``('a, 'b)`` t -> unit

Empty a hash table and shrink the size of the bucket table to its initial size.

  • since 4.00.0
val copy : ``('a, 'b)`` t -> ``('a, 'b)`` t

Return a copy of the given hashtable.

val add : ``('a, 'b)`` t -> 'a -> 'b -> unit

Hashtbl.add tbl key data adds a binding of key to data in table tbl. Previous bindings for key are not removed, but simply hidden. That is, after performing remove tbl key, the previous binding for key, if any, is restored. (Same behavior as with association lists.)

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

Hashtbl.find tbl x returns the current binding of x in tbl, or raises Not_found if no such binding exists.

val find_opt : ``('a, 'b)`` t -> 'a -> 'b option

Hashtbl.find_opt tbl x returns the current binding of x in tbl, or None if no such binding exists.

  • since 4.05
val find_all : ``('a, 'b)`` t -> 'a -> 'b list

Hashtbl.find_all tbl x returns the list of all data associated with x in tbl. The current binding is returned first, then the previous bindings, in reverse order of introduction in the table.

val mem : ``('a, 'b)`` t -> 'a -> bool

Hashtbl.mem tbl x checks if x is bound in tbl.

val remove : ``('a, 'b)`` t -> 'a -> unit

Hashtbl.remove tbl x removes the current binding of x in tbl, restoring the previous binding if it exists. It does nothing if x is not bound in tbl.

val replace : ``('a, 'b)`` t -> 'a -> 'b -> unit

Hashtbl.replace tbl key data replaces the current binding of key in tbl by a binding of key to data. If key is unbound in tbl, a binding of key to data is added to tbl. This is functionally equivalent to remove tbl key followed by add tbl key data.

val iter : ``('a -> 'b ->unit)``-> ``('a, 'b)`` t -> unit

Hashtbl.iter f tbl applies f to all bindings in table tbl. f receives the key as first argument, and the associated value as second argument. Each binding is presented exactly once to f.

The order in which the bindings are passed to f is unspecified. However, if the table contains several bindings for the same key, they are passed to f in reverse order of introduction, that is, the most recent binding is passed first.

If the hash table was created in non-randomized mode, the order in which the bindings are enumerated is reproducible between successive runs of the program, and even between minor versions of OCaml. For randomized hash tables, the order of enumeration is entirely random.

The behavior is not specified if the hash table is modified by f during the iteration.

val filter_map_inplace : ``('a -> 'b -> 'boption``)``-> ``('a, 'b)`` t -> unit

Hashtbl.filter_map_inplace f tbl applies f to all bindings in table tbl and update each binding depending on the result of f. If f returns None, the binding is discarded. If it returns Some new_val, the binding is update to associate the key to new_val.

Other comments for iter apply as well.

  • since 4.03.0
val fold : ``('a -> 'b -> 'c -> 'c)`` -> ``('a, 'b)`` t -> 'c -> 'c

Hashtbl.fold f tbl init computes (f kN dN ... (f k1 d1 init)...), where k1 ... kN are the keys of all bindings in tbl, and d1 ... dN are the associated values. Each binding is presented exactly once to f.

The order in which the bindings are passed to f is unspecified. However, if the table contains several bindings for the same key, they are passed to f in reverse order of introduction, that is, the most recent binding is passed first.

If the hash table was created in non-randomized mode, the order in which the bindings are enumerated is reproducible between successive runs of the program, and even between minor versions of OCaml. For randomized hash tables, the order of enumeration is entirely random.

The behavior is not specified if the hash table is modified by f during the iteration.

val length : ``('a, 'b)`` t -> int

Hashtbl.length tbl returns the number of bindings in tbl. It takes constant time. Multiple bindings are counted once each, so Hashtbl.length gives the number of times Hashtbl.iter calls its first argument.

valrandomize : ``unit-> unit

After a call to Hashtbl.randomize(), hash tables are created in randomized mode by default: create returns randomized hash tables, unless the ~random:false optional parameter is given. The same effect can be achieved by setting the R parameter in the OCAMLRUNPARAM environment variable.

It is recommended that applications or Web frameworks that need to protect themselves against the denial-of-service attack described in create call Hashtbl.randomize() at initialization time.

Note that once Hashtbl.randomize() was called, there is no way to revert to the non-randomized default behavior of create. This is intentional. Non-randomized hash tables can still be created using Hashtbl.create ~random:false.

  • since 4.00.0
valis_randomized : ``unit-> bool

Return true if the tables are currently created in randomized mode by default, false otherwise.

  • since 4.03.0
valrebuild : ``?random:bool-> ``('a, 'b)`` t -> ``('a, 'b)`` t

Return a copy of the given hashtable. Unlike copy, rebuild h re-hashes all the (key, value) entries of the original table h. The returned hash table is randomized if h was randomized, or the optional random parameter is true, or if the default is to create randomized hash tables; see create for more information.

rebuild can safely be used to import a hash table built by an old version of the Hashtbl module, then marshaled to persistent storage. After unmarshaling, apply rebuild to produce a hash table for the current version of the Hashtbl module.

  • since 4.12.0
type statistics`` = ``{

    

  1. 

num_bindings : int;


(*
Number of bindings present in the table. Same value as returned by
length.


*)

   
    

  1. 

num_buckets : int;


(*
Number of buckets in the table.


*)

   
    

  1. 

max_bucket_length : int;


(*
Maximal number of bindings per bucket.


*)

   
    

  1. 

bucket_histogram : ``int array``;


(*
Histogram of bucket sizes. This array histo has length
max_bucket_length + 1. The value of histo.(i) is the number of
buckets whose size is i.


*)

   
}
  • since 4.00.0
val stats : ``('a, 'b)`` t -> statistics

Hashtbl.stats tbl returns statistics about the table tbl: number of buckets, size of the biggest bucket, distribution of buckets by size.

  • since 4.00.0

Hash tables and Sequences

val to_seq : ``('a, 'b)`` t -> ``('a*'b)`` Seq.t

Iterate on the whole table. The order in which the bindings appear in the sequence is unspecified. However, if the table contains several bindings for the same key, they appear in reversed order of introduction, that is, the most recent binding appears first.

The behavior is not specified if the hash table is modified during the iteration.

  • since 4.07
val to_seq_keys : ``('a, _)`` t -> 'a Seq.t

Same as Seq.map fst (to_seq m)

  • since 4.07
val to_seq_values : ``(_, 'b)`` t -> 'b Seq.t

Same as Seq.map snd (to_seq m)

  • since 4.07
val add_seq : ``('a, 'b)`` t -> ``('a*'b)`` Seq.t -> unit

Add the given bindings to the table, using add

  • since 4.07
val replace_seq : ``('a, 'b)`` t -> ``('a*'b)`` Seq.t -> unit

Add the given bindings to the table, using replace

  • since 4.07
val of_seq : ``('a*'b)`` Seq.t -> ``('a, 'b)`` t

Build a table from the given bindings. The bindings are added in the same order they appear in the sequence, using replace_seq, which means that if two pairs have the same key, only the latest one will appear in the table.

  • since 4.07

Functorial interface

The functorial interface allows the use of specific comparison and hash functions, either for performance/security concerns, or because keys are not hashable/comparable with the polymorphic builtins.

For instance, one might want to specialize a table for integer keys:

Copy 
module IntHash =
  struct
    type t = int
    let equal i j = i=j
    let hash i = i land max_int
  end

module IntHashtbl = Hashtbl.Make(IntHash)

let h = IntHashtbl.create 17 in
IntHashtbl.add h 12 "hello"

This creates a new module IntHashtbl, with a new type 'a IntHashtbl.t of tables from int to 'a. In this example, h contains string values so its type is string IntHashtbl.t.

Note that the new type 'a IntHashtbl.t is not compatible with the type ('a,'b) Hashtbl.t of the generic interface. For example, Hashtbl.length h would not type-check, you must use IntHashtbl.length.

module type HashedType=sig...end

The input signature of the functor Make.

module type S=sig...end

The output signature of the functor Make.

module Make (H:HashedType) : S with type key=H.t

Functor building an implementation of the hashtable structure. The functor Hashtbl.Make returns a structure containing a type key of keys and a type 'a t of hash tables associating data of type 'a to keys of type key. The operations perform similarly to those of the generic interface, but use the hashing and equality functions specified in the functor argument H instead of generic equality and hashing. Since the hash function is not seeded, the create operation of the result structure always returns non-randomized hash tables.

module type SeededHashedType=sig...end

The input signature of the functor MakeSeeded.

module type SeededS=sig...end

The output signature of the functor MakeSeeded.

Functor building an implementation of the hashtable structure. The functor Hashtbl.MakeSeeded returns a structure containing a type key of keys and a type 'a t of hash tables associating data of type 'a to keys of type key. The operations perform similarly to those of the generic interface, but use the seeded hashing and equality functions specified in the functor argument H instead of generic equality and hashing. The create operation of the result structure supports the ~``random optional parameter and returns randomized hash tables if ~random:true is passed or if randomization is globally on (see Hashtbl.randomize).

The polymorphic hash functions

valhash :'a -> int

Hashtbl.hash x associates a nonnegative integer to any value of any type. It is guaranteed that if x = y or Stdlib.compare x y = 0, then hash x = hash y. Moreover, hash always terminates, even on cyclic structures.

valseeded_hash : ``int-> 'a -> int

A variant of hash that is further parameterized by an integer seed.

  • since 4.00.0
valhash_param : ``int->``int-> 'a -> int

Hashtbl.hash_param meaningful total x computes a hash value for x, with the same properties as for hash. The two extra integer parameters meaningful and total give more precise control over hashing. Hashing performs a breadth-first, left-to-right traversal of the structure x, stopping after meaningful meaningful nodes were encountered, or total nodes (meaningful or not) were encountered. If total as specified by the user exceeds a certain value, currently 256, then it is capped to that value. Meaningful nodes are: integers; floating-point numbers; strings; characters; booleans; and constant constructors. Larger values of meaningful and total means that more nodes are taken into account to compute the final hash value, and therefore collisions are less likely to happen. However, hashing takes longer. The parameters meaningful and total govern the tradeoff between accuracy and speed. As default choices, hash and seeded_hash take meaningful = 10 and total = 100.

valseeded_hash_param : ``int->``int->``int-> 'a -> int

A variant of hash_param that is further parameterized by an integer seed. Usage: Hashtbl.seeded_hash_param meaningful total seed x.

  • since 4.00.0

More from the DkSDK Book

    1. DkSDK
      1. Package capnp
        1. Module Capnp
        1. Module Capnp_unix
      1. Package cmdliner
        1. Module Cmdliner
        1. Module Cmdliner_arg
        1. Module Cmdliner_base
        1. Module Cmdliner_cline
        1. Module Cmdliner_cmd
        1. Module Cmdliner_docgen
        1. Module Cmdliner_eval
        1. Module Cmdliner_info
        1. Module Cmdliner_manpage
        1. Module Cmdliner_msg
        1. Module Cmdliner_term
        1. Module Cmdliner_term_deprecated
        1. Module Cmdliner_trie
      1. Package fmt
        1. Module Fmt
        1. Module Fmt_cli
        1. Module Fmt_tty
      1. Package logs
        1. Module Logs
        1. Module Logs_cli
        1. Module Logs_fmt
        1. Module Logs_lwt
        1. Module Logs_threaded
      1. Package lwt
        1. Module Lwt
        1. Module Lwt_bytes
        1. Module Lwt_condition
        1. Module Lwt_config
        1. Module Lwt_engine
        1. Module Lwt_features
        1. Module Lwt_fmt
        1. Module Lwt_gc
        1. Module Lwt_io
        1. Module Lwt_list
        1. Module Lwt_main
        1. Module Lwt_mutex
        1. Module Lwt_mvar
        1. Module Lwt_pool
        1. Module Lwt_pqueue
        1. Module Lwt_preemptive
        1. Module Lwt_process
        1. Module Lwt_result
        1. Module Lwt_seq
        1. Module Lwt_sequence
        1. Module Lwt_stream
        1. Module Lwt_switch
        1. Module Lwt_sys
        1. Module Lwt_throttle
        1. Module Lwt_timeout
        1. Module Lwt_unix
      1. Package mtime
        1. Module Mtime
        1. Module Mtime_clock
      1. Package ocaml
        1. Module Bigarray
        1. Module Condition
        1. Module Dynlink
        1. Module Event
        1. Module Mutex
        1. Module Profiling
        1. Module Semaphore
        1. Module Stdlib
          1. ...
          1. Module Stdlib.Hashtbl
            1. Module Hashtbl.Make
            1. Module Hashtbl.MakeSeeded
        1. Module Str
        1. Module Thread
        1. Module ThreadUnix
        1. Module Topdirs
        1. Module Unix
        1. Module UnixLabels
      1. Package
      1. Package result
        1. Module Result
      1. Package stdint
        1. Module Stdint
Logo for Diskuv

The development kit to create safe software and products.