[Chicken-janitors] #1290: Unit srfi-69: does not specify action when two merged hash tables contain the same keys

[Chicken Trac]

#1290: Unit srfi-69: does not specify action when two merged hash tables contain
the same keys
------------------------+---------------------
 Reporter:  nxg         |      Owner:
     Type:  defect      |     Status:  new
 Priority:  minor       |  Milestone:  someday
Component:  extensions  |    Version:  4.10.x
 Keywords:  srfi-69     |
------------------------+---------------------
 The documentation for `hash-table-merge` says

     (hash-table-merge HASH-TABLE-1 HASH-TABLE-2) procedure
     Returns a new HASH-TABLE with the union of HASH-TABLE-1 and HASH-
 TABLE-2.

 However this does not specify which hash table 'wins' when the two hash
 tables contain the same key.  This procedure doesn't appear in SRFI 69, so
 that doesn't resolve it.

 From experiment, and by inspection of the code, it is clear that hash-
 table-1 wins:

 {{{
     % csi

     CHICKEN
     (c) 2008-2015, The CHICKEN Team
     (c) 2000-2007, Felix L. Winkelmann
     Version 4.10.0 (rev b259631)
     macosx-unix-clang-x86-64 [ 64bit manyargs dload ptables ]
     compiled 2015-08-04 on yves.more-magic.net (Linux)

     #;1> (use srfi-69)
     ; loading /Data/tools/chicken-4.10.0/lib/chicken/7/srfi-69.import.so
 ...
     ; loading library srfi-69 ...
     #;2> (define h1 (alist->hash-table '((a . 1) (x . 2))))
     #;3> (define h2 (alist->hash-table '((b . 1) (x . 3))))
     #;4> (define h3 (hash-table-merge h1 h2))
     #;5> (hash-table->alist h3)
     ((x . 2) (b . 1) (a . 1))
     #;6>
 }}}

 But it would be good to document this, so that the user can know this is
 not an unspecified behaviour which may change in future.  The same goes
 for

 {{{
 (alist->hash-table '((a . 1) (a . 2)))
 }}}

 Does this produce a hash table with `a` defined as 1 or 2 (it's 1, but the
 documentation doesn't guarantee this)

 ----

 Incidentally (and at the risk of putting distinct notes into one ticket),
 it would be really attractive if this library defined a pure-functional
 `hash-table-set` which mapped `(hash-table? any any) -> hash-table?`

--
Ticket URL: <http://bugs.call-cc.org/ticket/1290>
CHICKEN Scheme <http://www.call-cc.org/>
CHICKEN Scheme is a compiler for the Scheme programming language.