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Reading and Writing Structures

Problem

You want to save a structure to disk and read it at a later time.

Solution

Print convert the structure before you write to disk.

When you read it again use read followed by eval.

> (require (lib "pconvert.ss"))
> (define-struct user (name password) (make-inspector))
> (define guest (make-user "Anonymous" "1234"))
> guest
#<struct:user>
> (constructor-style-printing #t)
> (print-convert guest)
(make-user "Anonymous" "1234")

> (let ([output (open-output-string)])
    (write (print-convert guest) output)
    (get-output-string output))
"(make-user \"Anonymous\" \"1234\")"    

> (read (open-input-string "(make-user \"Anonymous\" 1234)"))
(make-user "Anonymous" 1234)            

> (eval (read (open-input-string "(make-user \"Anonymous\" 1234)")))
#<struct:user>

Discussion

Note that we make an inspector to "see inside" the structure otherwise print-convert can't see the contents of the structure.

Note that print-convert is in general useful for serialization purposes.

See Also

IdiomSerializeToFile

Comments about this recipe

Please remember that when further converting a print-converted list into a string with, say, format, you need to use "~s" instead of "~a" in the format string (so that you write the value instead of displaying it).

See http://list.cs.brown.edu/pipermail/plt-scheme/2005-March/008262.html for a relevant discussion thread.

-- DanielSilva - 23 Mar 2005

Contributors

-- JensAxelSoegaard - 04 Jun 2004