In chapter 4 of the book, Successful Lisp, by David B. Lamkins, there is a simple application to keep track of bank checks.
https://dept-info.labri.fr/~strandh/Teaching/MTP/Common/David-Lamkins/chapter04.html
At the end, we write a macro that will save and restore functions. The problem occurs when I execute the reader function and the value to read is the hash table.
The macro to save and restore functions is:
(defmacro def-i/o (writer-name reader-name (&rest vars))
(let ((file-name (gensym))
(var (gensym))
(stream (gensym)))
`(progn
(defun ,writer-name (,file-name)
(with-open-file (,stream ,file-name
:direction :output :if-exists :supersede)
(dolist (,var (list ,@vars))
(declare (special ,@vars))
(print ,var ,stream))))
(defun ,reader-name (,file-name)
(with-open-file (,stream ,file-name
:direction :input :if-does-not-exist :error)
(dolist (,var ',vars)
(set ,var (read ,stream)))))
t)))
Here is my hash table and what is going on:
(defvar *payees* (make-hash-table :test #'equal))
(check-check 100.00 "Acme" "Rocket booster T-1000")
CL-USER> *payees*
#<HASH-TABLE :TEST EQUAL :COUNT 0 {25520F91}>
CL-USER> (check-check 100.00 "Acme" "T-1000 rocket booster")
#S(CHECK
:NUMBER 100
:DATE "2020-4-1"
:AMOUNT 100.0
:PAID "Acme"
:MEMO "T-1000 rocket booster")
CL-USER> (def-i/o save-checks charge-checks (*payees*))
T
CL-USER> (save-checks "/home/checks.dat")
NIL
CL-USER> (makunbound '*payees*)
*PAYEES*
CL-USER> (load-checks "/home/checks.dat")
; Evaluation aborted on #<SB-INT:SIMPLE-READER-ERROR "illegal sharp macro character: ~S" {258A8541}>.
In Lispworks, the error message is:
Error: subcharacter #\< not defined for dispatch char #\#.
To simplify, I get the same error if I execute:
CL-USER> (defvar *payees* (make-hash-table :test #'equal))
*PAYEES*
CL-USER> (with-open-file (in "/home/checks.dat"
:direction :input)
(set *payees* (read in)))
; Evaluation aborted on #<SB-INT:SIMPLE-READER-ERROR "illegal sharp macro character: ~S" {23E83B91}>.
Can someone explain to me where the problem is coming from, and what I need to fix in my code for this to work. Thank you in advance for the explanations you can give me and the help you can give me.
Values which cannot be read back are printed with #<, see Sharpsign Less-Than-Sign. Common Lisp does not define how hash-tables should be printed readably (there is no reader syntax for hash tables):
* (make-hash-table)
#<HASH-TABLE :TEST EQL :COUNT 0 {1006556E53}>
The example in the book is only suitable to be used with the bank example that was previously shown, and is not intended to be a general-purpose serialization mechanism.
There are many ways to print a hash-table and read them back, depending on your needs, but there is no default representation. See cl-store for a library that store arbitrary objects.
dump functionLet's write a form that evaluate to an equivalent hash-table; let's define a generic function named dump, and a default method that simply returns the object as-is:
(defgeneric dump (object)
(:method (object) object))
Given a hash-table, we can serialize it as a plist (sequence of key/value elements in a list), while also calling dump on keys and values, in case our hash-tables contain hash-tables:
(defun hash-table-plist-dump (hash &aux plist)
(with-hash-table-iterator (next hash)
(loop
(multiple-value-bind (some key value) (next)
(unless some
(return plist))
(push (dump value) plist)
(push (dump key) plist)))))
Instead of reinventing the wheel, we could also have used hash-table-plist from the alexandria system.
(ql:quickload :alexandria)
The above is equivalent to:
(defun hash-table-plist-dump (hash)
(mapcar #'dump (alexandria:hash-table-plist hash)))
Then, we can specialize dump for hash-tables:
(defmethod dump ((hash hash-table))
(loop
for (initarg accessor)
in '((:test hash-table-test)
(:size hash-table-size)
(:rehash-size hash-table-rehash-size)
(:rehash-threshold hash-table-rehash-threshold))
collect initarg into args
collect `(quote ,(funcall accessor hash)) into args
finally (return
(alexandria:with-gensyms (h k v)
`(loop
:with ,h := (make-hash-table ,@args)
:for (,k ,v)
:on (list ,@(hash-table-plist-dump hash))
:by #'cddr
:do (setf (gethash ,k ,h) ,v)
:finally (return ,h))))))
The first part of the loop computes all the hash-table properties, like the kind of hash function to use or the rehash-size, and builds args, the argument list for a call to make-hash-table with the same values.
In the finally clause, we build a loop expression (see backquotes) that first allocates the hash-table, then populates it according to the current values of the hash, and finally return the new hash. Note that the generated code does not depend on alexandria, it could be read back from another Lisp system that does not have this dependency.
CL-USER> (alexandria:plist-hash-table '("abc" 0 "def" 1 "ghi" 2 "jkl" 3)
:test #'equal)
#<HASH-TABLE :TEST EQUAL :COUNT 4 {100C91F8C3}>
Dump it:
CL-USER> (dump *)
(LOOP :WITH #:H603 := (MAKE-HASH-TABLE :TEST 'EQUAL :SIZE '14 :REHASH-SIZE '1.5
:REHASH-THRESHOLD '1.0)
:FOR (#:K604 #:V605) :ON (LIST "jkl" 3 "ghi" 2 "def" 1 "abc"
0) :BY #'CDDR
:DO (SETF (GETHASH #:K604 #:H603) #:V605)
:FINALLY (RETURN #:H603))
The generated data is also valid Lisp code, evaluate it:
CL-USER> (eval *)
#<HASH-TABLE :TEST EQUAL :COUNT 4 {100CD5CE93}>
The resulting hash is equalp to the original one:
CL-USER> (equalp * ***)
T