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This SRFI defines an interface to hash tables, which are widely recognized as a fundamental data structure for a wide variety of applications. A hash table is a data structure that:
Hash tables themselves don't really need defending: almost all dynamically typed languages, from awk to JavaScript to Lua to Perl to Python to Common Lisp, and including many Scheme implementations, provide them in some form as a fundamental data structure. Therefore, what needs to be defended is not the data structure but the procedures. This SRFI is at an intermediate level. It supports a great many convenience procedures on top of the basic hash table interfaces provided by SRFI 69 and R6RS. Nothing in it adds power to what those interfaces provide, but it does add convenience in the form of pre-debugged routines to do various common things, and even some things not so commonly done but useful.
There is no mandated support for thread safety, immutability, or weakness, though there are portable hooks for specifying these features.
This specification accepts separate equality predicates and hash functions for backward compatibility, but strongly encourages the use of SRFI 128 comparators, which package a type test, an equality predicate, and a hash function into a single bundle.
This SRFI is downward compatible with SRFI 69. Some procedures have been given new preferred names for compatibility with other SRFIs, but in all cases the SRFI 69 names have been retained as deprecated synonyms; implementations must still provide them, however. They appear in this SRFI in small print.
There is one absolute incompatibility with SRFI 69: the reflective procedure
hash-table-hash-function may return #f,
which is not permitted by SRFI 69. See the specification for details.
The relatively few hash table procedures in R6RS are all available
in this SRFI under somewhat different names. The only substantive
difference is that R6RS hashtable-values and
hashtable-entries return vectors, whereas in this
SRFI hash-table-values and hash-table-entries
return lists. This SRFI adopts SRFI 69's term hash-table
rather than R6RS's hashtable, because of the universal
use of "hash table" rather than "hashtable" in other computer languages and
in technical prose generally. Besides, the English word
hashtable obviously means something that can be ... hashted.
In addition, the hashtable-ref and
hashtable-update! of R6RS correspond to the
hash-table-ref/default and
hash-table-update!/default of both SRFI 69 and this SRFI.
It would be trivial to provide the R6RS names on top of this SRFI.
As usual, the Common Lisp names are completely different from the Scheme names. Common Lisp provides the following capabilities that are not in this SRFI:
equalp
(which does not exist in Scheme).
With-hash-table-iterator is a hash
table external iterator implemented as a local macro.
Sxhash is a implementation-specific
hash function for the equal predicate.
It has the property that objects in different instantiations
of the same Lisp implementation that are
similar,
a concept analogous to equal but defined across all
instantiations,
always return the same value from sxhash; for example,
the symbol
xyz will have the same sxhash result in
all instantiations.
The procedures in this SRFI are drawn primarily from SRFI 69 and R6RS. In addition, the following sources are acknowledged:
hash-table-mutable? procedure and the
second argument of hash-table-copy (which allows
the creation of immutable hash tables) are from R6RS, renamed
in the style of this SRFI.
hash-table-intern! procedure is from
Racket,
renamed in the style of this SRFI.
hash-table-find procedure is a modified
version of table-search in
Gambit.
hash-table-unfold and
hash-table-count were suggested by
SRFI 1.
hash-table=? and
hash-table-map were suggested by
Haskell's Data.Map.Strict module.
hash-table-map->list is from
Guile.
The procedures hash-table-empty?,
hash-table-empty-copy, hash-table-pop!,
hash-table-map!, hash-table-intersection!,
hash-table-difference!, and hash-table-xor!
were added for convenience and completeness.
The native hash tables of MIT, SISC, Bigloo, Scheme48, SLIB, RScheme, Scheme 7, Scheme 9, Rep, and FemtoLisp were also investigated, but no additional procedures were incorporated.
The slash in the names of some procedures can be pronounced "with".
The procedures in this SRFI are in the (srfi 125) library
(or (srfi :125) on R6RS).
All references to "executing in expected amortized constant time" presuppose that a satisfactory hash function is available. Arbitrary or impure hash functions can make a hash of any implementation.
Hash tables are allowed to cache the results of calling the equality predicate and hash function, so programs cannot rely on the hash function being called exactly once for every primitive hash table operation: it may be called zero, one, or more times.
It is an error if the procedure argument of hash-table-find,
hash-table-count, hash-table-map,
hash-table-for-each, hash-table-map!,
hash-table-map->list, hash-table-fold
or hash-table-prune! mutates the hash table being walked.
It is an error to pass two hash tables that have different comparators or equality predicates to any of the procedures of this SRFI.
Implementations are permitted to ignore user-specified hash
functions in certain circumstances. Specifically, if the
equality predicate, whether passed as part of a comparator
or explicitly, is more fine-grained (in the sense of R7RS-small
section 6.1) than equal?, the implementation is
free — indeed, is encouraged — to ignore the user-specified
hash function and use something implementation-dependent.
This allows the use of addresses as hashes, in which case
the keys must be rehashed if they are moved by the garbage
collector. Such a hash function is unsafe to use outside
the context of implementation-provided hash tables. It can
of course be exposed by an implementation as an extension,
with suitable warnings against inappropriate uses.
It is an error to mutate a key during or after its insertion into a hash table in such a way that the hash function of the table will return a different result when applied to that key.
make-hash-table, hash-table,
hash-table-unfold, alist->hash-table
hash-table?, hash-table-contains?,
hash-table-exists? (deprecated),
hash-table-empty?, hash-table=?,
hash-table-mutable?
hash-table-ref, hash-table-ref/default
hash-table-set!,
hash-table-delete!,
hash-table-intern!, hash-table-update!,
hash-table-update!/default,
hash-table-pop!, hash-table-clear!
hash-table-size, hash-table-keys,
hash-table-values, hash-table-entries,
hash-table-find, hash-table-count
hash-table-map, hash-table-for-each,
hash-table-walk (deprecated),
hash-table-map!, hash-table-map->list,
hash-table-fold, hash-table-prune!
hash-table-copy, hash-table-empty-copy,
hash-table->alist
hash-table-union!,
hash-table-merge! (deprecated),
hash-table-intersection!,
hash-table-difference!, hash-table-xor!
hash, string-hash, string-ci-hash,
hash-by-identity, hash-table-equivalence-function,
hash-table-hash-function
(make-hash-table comparator [ arg ... ])
(make-hash-table equality-predicate [ hash-function ] [ arg ... ])
Returns a newly allocated hash table whose equality predicate and hash function are extracted from comparator. Alternatively, for backward compatibility with SRFI 69 the equality predicate and hash function can be passed as separate arguments; this usage is deprecated.
As mentioned above, implementations are free to use an appropriate
implementation-dependent hash function instead of the
specified hash function, provided that the specified equality predicate
is a refinement of the equal? predicate.
This applies whether the hash function and equality predicate are passed
as separate arguments or packaged up into a comparator.
If an equality predicate rather than a comparator is provided,
the ability to omit the hash-function argument is severely
limited. The implementation must provide hash functions appropriate
for use with the predicates eq?, eqv?,
equal?, string=?, and string-ci=?,
and may extend this list. But if any unknown equality predicate is
provided without a hash function, an error should be signaled.
The constraints on equality predicates and hash functions are given in
SRFI 128.
The meaning of any further arguments is implementation-dependent.
However, implementations which support the ability to specify the
initial capacity of a hash table should interpret a non-negative
exact integer as the specification of that capacity. In addition,
if the symbols thread-safe, weak-keys,
ephemeral-keys, weak-values, or
ephemeral-values are present, implementations
should create thread-safe hash tables, hash tables with weak
keys or ephemeral keys, or hash tables with weak or ephemeral
values respectively. Implementations are free to use ephemeral
keys or values when weak keys or values respectively have been
requested. To avoid collision with the hash-function
argument, none of these arguments can be procedures.
(R6RS make-eq-hashtable, make-eqv-hashtable,
and make-hashtable; Common Lisp make-hash-table)
(hash-table comparator [ key value ] ...)
Returns a newly allocated hash table, created as if by
make-hash-table using comparator.
For each pair of arguments, an association is added to the
new hash table with key as its key and value
as its value. If the implementation supports immutable hash
tables, this procedure returns an immutable hash table.
If the same key (in the sense of the equality predicate) is
specified more than once, it is an error.
(hash-table-unfold stop? mapper successor seed comparator arg ... )
Create a new hash table as if by make-hash-table using
comparator and the args. If the result of applying
the predicate stop? to seed is true, return the hash
table. Otherwise, apply the procedure mapper to seed.
Mapper returns two values, which are inserted into the hash
table as the key and the value respectively. Then get a new seed by
applying the procedure successor to seed, and repeat
this algorithm.
(alist->hash-table alist comparator arg ...)
(alist->hash-table alist equality-predicate [ hash-function ] arg ...)
Returns a newly allocated hash-table as if by make-hash-table
using comparator and the args. It is then initialized
from the associations of alist. Associations earlier in the
list take precedence over those that come later. The second form is
for compatibility with SRFI 69, and is deprecated.
(hash-table? obj)
Returns #t if obj is a hash table, and
#f otherwise. (R6RS hashtable?;
Common Lisp hash-table-p)
(hash-table-contains? hash-table key)
(hash-table-exists? hash-table key)
Returns #t if there is any association to key
in hash-table, and #f otherwise. Must execute
in expected amortized constant time. The hash-table-exists?
procedure is the same as hash-table-contains?, is provided
for backward compatibility with SRFI 69, and is deprecated.
(R6RS hashtable-contains?)
(hash-table-empty? hash-table)
Returns #t if hash-table contains no associations,
and #f otherwise.
(hash-table=? value-comparator hash-table1 hash-table2)
Returns #t if hash-table1 and
hash-table2 have the same keys (in the sense
of their common equality predicate) and each key has the same
value (in the sense of value-comparator), and
#f otherwise.
(hash-table-mutable? hash-table)
Returns #t if the hash table is mutable.
Implementations may or may not support immutable hash tables.
(R6RS hashtable-mutable?)
The following procedures, given a key, return the corresponding value.
(hash-table-ref hash-table key [ failure [ success ] ])
Extracts the value associated to key in hash-table, invokes the procedure success on it, and returns its result; if success is not provided, then the value itself is returned. If key is not contained in hash-table and failure is supplied, then failure is invoked on no arguments and its result is returned. Otherwise, it is an error. Must execute in expected amortized constant time, not counting the time to call the procedures. SRFI 69 does not support the success procedure.
(hash-table-ref/default hash-table key default)
Semantically equivalent to, but may be more efficient than, the following code:
(hash-table-refhash-table key(lambda ()default))
(R6RS hashtable-ref; Common Lisp gethash)
The following procedures alter the associations in a hash table either unconditionally, or conditionally on the presence or absence of a specified key. It is an error to add an association to a hash table whose key does not satisfy the type test predicate of the comparator used to create the hash table.
(hash-table-set! hash-table arg ...)
Repeatedly mutates hash-table, creating new associations
in it by processing the arguments from left to right.
The args alternate between keys and values. Whenever
there is a previous association for a key, it is deleted. It is
an error if the type check procedure of the comparator of
hash-table, when invoked on a key, does not return
#t. Likewise, it is an error if a key is not a
valid argument to the equality predicate of hash-table.
Returns an unspecified value. Must execute in expected amortized
constant time per key. SRFI 69, R6RS hashtable-set!
and Common Lisp (setf gethash) do not handle multiple
associations.
(hash-table-delete! hash-table key ...)
Deletes any association to each key in hash-table
and returns the number of keys that had associations. Must execute
in expected amortized constant time per key. SRFI 69, R6RS
hashtable-delete!, and Common Lisp remhash
do not handle multiple associations.
(hash-table-intern! hash-table key failure)
Effectively invokes hash-table-ref with the given
arguments and returns what it returns. If key was not
found in hash-table, its value is set to the result of
calling failure. Must execute in expected amortized constant time.
(hash-table-update! hash-table key updater [ failure [ success ] ])
Semantically equivalent to, but may be more efficient than, the following code:
(hash-table-set!hash-table key(updater(hash-table-refhash-table key failure success)))
Must execute in expected amortized constant time. Returns an
unspecified value. (SRFI 69 and R6RS hashtable-update!
do not support the success procedure)
(hash-table-update!/default hash-table key updater default)
Semantically equivalent to, but may be more efficient than, the following code:
(hash-table-set!hash-table key(updater(hash-table-ref/defaulthash-table key default)))
Must execute in expected amortized constant time. Returns an unspecified value.
(hash-table-pop! hash-table)
Chooses an arbitrary association from hash-table and removes it, returning the key and value as two values.
It is an error if hash-table is empty.
(hash-table-clear! hash-table)
Delete all the associations from hash-table.
(R6RS hashtable-clear!; Common Lisp clrhash)
These procedures process the associations of the hash table in an unspecified order.
(hash-table-size hash-table)
Returns the number of associations in hash-table as an
exact integer. Should execute in constant time.
(R6RS hashtable-size; Common Lisp hash-table-count.)
(hash-table-keys hash-table)
Returns a newly allocated list of all the keys in hash-table.
R6RS hashtable-keys returns a vector.
(hash-table-values hash-table)
Returns a newly allocated list of all the keys in hash-table.
(hash-table-entries hash-table)
Returns two values, a newly allocated list of all the keys in
hash-table and a newly allocated list of all the values
in hash-table in the corresponding order. R6RS
hash-table-entries returns vectors.
(hash-table-find proc hash-table failure)
For each association of hash-table, invoke proc
on its key and value. If proc returns true, then
hash-table-find returns what proc returns.
If all the calls to proc return #f, return
the result of invoking the thunk failure.
(hash-table-count pred hash-table)
For each association of hash-table, invoke pred on its key and value. Return the number of calls to pred which returned true.
These procedures process the associations of the hash table in an unspecified order.
(hash-table-map proc comparator hash-table)
Returns a newly allocated hash table as if by
(make-hash-table comparator).
Calls proc for every association in hash-table
with the value of the association. The key of the association
and the result of invoking proc are entered into the
new hash table. Note that this is not the result of
lifting mapping over the domain of hash tables, but it is
considered more useful.
If comparator recognizes multiple keys in the hash-table as equivalent, any one of such associations is taken.
(hash-table-for-each proc hash-table)
(hash-table-walk hash-table proc)
Calls proc for every association in hash-table
with two arguments: the key of the association and the value of
the association. The value returned by proc is discarded.
Returns an unspecified value. The hash-table-walk
procedure is equivalent to hash-table-for-each with
the arguments reversed, is provided for backward compatibility
with SRFI 69, and is deprecated. (Common Lisp maphash)
(hash-table-map! proc hash-table)
Calls proc for every association in hash-table with two arguments: the key of the association and the value of the association. The value returned by proc is used to update the value of the association. Returns an unspecified value.
(hash-table-map->list proc hash-table)
Calls proc for every association in hash-table with two arguments: the key of the association and the value of the association. The values returned by the invocations of proc are accumulated into a list, which is returned.
(hash-table-fold proc seed hash-table)
(hash-table-fold hash-table proc seed)
Calls proc for every association in hash-table
with three arguments: the key of the association, the value of
the association, and an accumulated value val.
Val is seed for the first invocation of
procedure, and for subsequent invocations of proc,
the returned value of the previous invocation. The value returned
by hash-table-fold is the return value of the last
invocation of proc. The order of arguments with
hash-table as the first argument is provided for
SRFI 69 compatibility, and is deprecated.
(hash-table-prune! proc hash-table)
Calls proc for every association in hash-table with two arguments, the key and the value of the association, and removes all associations from hash-table for which proc returns true. Returns an unspecified value.
(hash-table-copy hash-table [ mutable? ])
Returns a newly allocated hash table with the same properties
and associations as hash-table. If the second argument
is present and is true, the new hash table is mutable. Otherwise
it is immutable provided that the implementation supports immutable
hash tables. SRFI 69 hash-table-copy does not support
a second argument. (R6RS hashtable-copy)
(hash-table-empty-copy hash-table)
Returns a newly allocated mutable hash table with the same properties as hash-table, but with no associations.
(hash-table->alist hash-table)
Returns an alist with the same associations as hash-table in an unspecified order.
(hash-table-union! hash-table1 hash-table2)
(hash-table-merge! hash-table1 hash-table2)
Adds the associations of hash-table2 to
hash-table1 and returns hash-table1.
If a key appears in both hash tables, its value is set to the value
appearing in hash-table1. Returns
hash-table1. The hash-table-merge!
procedure is the same as hash-table-union!, is provided
for compatibility with SRFI 69, and is deprecated.
(hash-table-intersection! hash-table1 hash-table2)
Deletes the associations from hash-table1 whose keys don't also appear in hash-table2 and returns hash-table1.
(hash-table-difference! hash-table1 hash-table2)
Deletes the associations of hash-table1 whose keys are also present in hash-table2 and returns hash-table1.
(hash-table-xor! hash-table1 hash-table2)
Deletes the associations of hash-table1 whose keys are also present in hash-table2, and then adds the associations of hash-table2 whose keys are not present in hash-table1 to hash-table1. Returns hash-table1.
These functions are made part of this SRFI solely for compatibility with SRFI 69, and are deprecated.
(hash obj [ arg ] )
The same as SRFI 128's default-hash procedure,
except that it must accept (and should ignore) an optional second argument.
(string-hash obj [ arg ] )
Similar to SRFI 128's string-hash procedure,
except that it must accept (and should ignore) an optional second argument.
It is incompatible with the procedure of the same name exported by SRFI 128
and SRFI 126.
(string-ci-hash obj [ arg ] )
Similar to SRFI 128's string-ci-hash procedure,
except that it must accept (and should ignore) an optional second argument.
It is incompatible with the procedure of the same name exported by SRFI 128
and SRFI 126.
(hash-by-identity obj [ arg ] )
The same as SRFI 128's default-hash procedure,
except that it must accept (and should ignore) an optional second argument.
However, if the implementation replaces the hash function associated
with the eq? predicate with an implementation-dependent
alternative, it is an error to call this procedure at all.
(hash-table-equivalence-function hash-table)
Returns the equivalence procedure used to create hash-table.
(hash-table-hash-function hash-table)
Returns the hash function used to create hash-table.
However, if the implementation has replaced the user-specified
hash function with an implementation-specific alternative, the
implementation may return #f instead.
The current sample implementation is in the code repository of this SRFI. It relies upon SRFI 126 and SRFI 128, but the sample implementation for SRFI 126 is easily layered over any hash table implementation that supports either SRFI 69 or R6RS.
The sample implementation of this SRFI never calls any hash function with two arguments.
The original intention was to make it possible to implement this SRFI on top of all the native hash table systems mentioned in Sources above as well. However, this turned out not to be practical for the following reasons:
eq?, eqv?, equal?, and
string=?.
equal? as the
equality predicate.
Native Guile hash tables are a special case. The equivalents of
hash-table-ref/default, hash-table-set!,
and hash-table-delete! require the equality predicate
and hash function to be passed to them explicitly (although there
are utility functions for eq?, eqv?,
and equal? hash tables). Consequently, hash tables
corresponding to this SRFI would have to be records containing a
Guile hash table, an equality predicate, and a hash function,
which means they could not interoperate directly with native
Guile hash tables.
Some of the language of this SRFI is copied from SRFI 69 with thanks to its author, Panu Kalliokoski. However, he is not responsible for what I have done with it. Thanks to Will Clinger for providing the sample implementation, and to Taylan Ulrich Bayırlı/Kammer for his spirited review.
I also acknowledge the members of the SRFI 125, 126, and 128 mailing lists, especially Takashi Kato, Alex Shinn, Shiro Kawai, and Per Bothner.
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The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
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