:hash-function behavior in make-hash-table inconsistent with documentation

[Jacek Złydach]

Version: SBCL 2.0.6 X86-64

Test case:

In the REPL, enter: (make-hash-table :hash-function nil)

Expected behavior:

A regular hash table, equivalent to one created by plain (make-hash-table), OR documentation directly mentioning that the argument MUST be a valid function designator if it's explicitly specified.

Observed behavior:

Condition of type UNDEFINED-FUNCTION: The function COMMON-LISP:NIL is undefined.

Context:

The SBCL 2.0.6 documentation for :hash-function argument states:

"If nil (the default), a hash function based on the test argument is used, which then must be one of the standardized hash table test functions, or one for which a default hash function has been defined using sb-ext:define-hash-table-test. If hash-function is specified, the test argument can be any two argument predicate consistent with it. The hash-function is expected to return a non-negative fixnum hash code."

The above text - in particular the beginning of the first sentence - seems to imply that explicit NIL value should be equivalent to not passing :hash-function argument at all. I believe that either NIL argument should be allowed, or the documentation should state explicitly that :hash-function must be a function designator.

Extra info:

- uname -a: Linux enterprise 5.3.0-59-generic #53~18.04.1-Ubuntu SMP Thu Jun 4 14:58:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
- *FEATURES*: (:SWANK :DEV :QUICKLISP :QUICKLISP-SUPPORT-HTTPS :ASDF3.3 :ASDF3.2 :ASDF3.1 :ASDF3 :ASDF2 :ASDF :OS-UNIX :NON-BASE-CHARS-EXIST-P :ASDF-UNICODE :ROS.INIT :X86-64 :GENCGC :64-BIT :ANSI-CL :COMMON-LISP :ELF :IEEE-FLOATING-POINT :LINUX :LITTLE-ENDIAN :PACKAGE-LOCAL-NICKNAMES :SB-CORE-COMPRESSION :SB-LDB :SB-PACKAGE-LOCKS :SB-THREAD :SB-UNICODE :SBCL :UNIX)

[Douglas Katzman (dougk)]

I'll reject NIL as a hash-function. It makes no sense, it's just opening a can of worms to suggest that there should be any kind of defaulting logic in these cases:
 (make-hash-table :test #'string= :hash-function nil) ; there is no default hashing for STRING=
 (make-hash-table :test #'equal :hash-function nil) ; default is known, but please don't do this
or
 (defun mytest (a b) (if (numberp a) (= a b) (eq a b))) ; a random user-defined test
 (make-hash-table :test #'mytest :hash-function) ; what could this even mean? I have no idea.