# Structure Polygdbm

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    (* Polygdbm -- GNU gdbm persistent polymorphic hashtables -- requires Dynlib *)

    type ('key, 'data) table

    exception NotFound
    exception AlreadyThere
    exception NotWriter
    exception Closed
    exception GdbmError of string

    val withtable  : string * Gdbm.openmode -> (('key, 'data) table -> 'a) -> 'a
    val add        : ('key, 'data) table -> 'key * 'data -> unit
    val insert     : ('key, 'data) table -> 'key * 'data -> unit
    val find       : ('key, 'data) table -> 'key -> 'data
    val peek       : ('key, 'data) table -> 'key -> 'data option
    val hasKey     : ('key, 'data) table -> 'key -> bool
    val remove     : ('key, 'data) table -> 'key -> unit
    val listKeys   : ('key, 'data) table -> 'key list
    val numItems   : ('key, 'data) table -> int
    val listItems  : ('key, 'data) table -> ('key * 'data) list
    val app        : ('key * 'data -> unit) -> ('key, 'data) table -> unit
    val map        : ('key * 'data -> 'a) -> ('key, 'data) table -> 'a list
    val fold       : ('key * 'data * 'a -> 'a) -> 'a -> ('key, 'data) table -> 'a
    val fastwrite  : bool ref
    val reorganize : ('key, 'data) table -> unit

    (*
       [('key, 'data) table] is the type of an opened table with keys of
       type 'key and associated values of type 'data.  The actual values
       of type 'key and 'data cannot contain function closures or abstract
       values.  Values involving references (even circular values) can be
       stored, but the identity of references is preserved only with every
       single key or value stored, not across several different values.

       The Polygdbm table files of are not portable across platforms,
       because word size and endianness affects the lay-out of values.

       A value of type table can be used only in the argument f to the
       withtable function.  This makes sure that the table is closed after
       use.

       [withtable (nam, mod) f] first opens the table db in file nam with
       mode mod, then applies f to db, then closes db.  Makes sure to
       close db even if an exception is raised during the evaluation of
       f(db).  Raises GdbmError with an informative message in case the
       table cannot be opened.  E.g. the table cannot be opened for
       reading if already opened for writing, and cannot be opened for
       writing if already opened for reading.

       [add db (k,v)] adds the pair (k, v) to db.  Raises AlreadyThere if
       there is a pair (k, _) in db already.  Raises NotWriter if db is
       not opened in write mode.

       [insert db (k, v)] adds the pair (k, v) to db, replacing any pair
       (k, _) at k if present.  Raises NotWriter if db is not opened in
       write mode.

       [find(db, k)] returns v if the pair (k, v) is in db; otherwise
       raises NotFound.

       [peek db k] returns SOME v if the pair (k, v) is in db; otherwise
       returns NONE.

       [hasKey(db, k)] returns true if there is a pair (k, _) in db;
       otherwise returns false.

       [remove db k] deletes the pair (k, _) from the table if present;
       otherwise raises NotFound.  Raises NotWriter if db is not opened in
       write mode.

       [listKeys db] returns a list of all keys in db in an unspecified
       order.

       [numItems db] is the number of (key, value) pairs in db.
       Equivalent to length(listKeys db).

       [listItems db] returns a list of all (key, value) pairs in db in some
       order.  Equivalent to
            List.map (fn key => (key, find(db,key))) (listKeys db)

       [app f db] is equivalent to List.app f (listItems db), provided the
       function f does not change the set of keys in the table.
       Otherwise the effect is unpredictable.

       [map f db] is equivalent to List.map f (listItems db), provided the
       function f does not change the set of keys in the table.
       Otherwise the result and effect are unpredictable.

       [fold f a db] is equivalent to
            List.foldr (fn ((k, v), r) => f(k, v, r)) a (listItems db)
       provided the function f does not change the set of keys in the
       table. Otherwise the result and effect are unpredictable.

       [fastwrite] can be set to speed up writes to a table.  By default,
       !fastwrite is false and every write to a table will be followed by
       file system synchronization.  This is safe, but slow if you perform
       thousands of writes.  However, if !fastwrite is true when calling
       withtable, then writes may not be followed by synchronization,
       which may speed up writes considerably.  In any case, the file
       system is synchronized before withtable returns.

       [reorganize db] has no visible effect, but may be called after a
       lot of deletions to shrink the size of the table file.
    *)

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