CODEWITHSUNDEEP
schemeevalracket
Jesse Millikan
Jesse Millikan

Reputation: 3145

Write Scheme data structures so they can be eval-d back in, or alternative

I'm writing an application (A juggling pattern animator) in PLT Scheme that accepts Scheme expressions as values for some fields. I'm attempting to write a small text editor that will let me "explode" expressions into expressions that can still be eval'd but contain the data as literals for manual tweaking.

For example,

(4hss->sexp "747")

is a function call that generates a legitimate pattern. If I eval and print that, it becomes 

(((7 3) - - -) (- - (4 2) -) (- (7 2) - -) (- - - (7 1)) ((4 0) - - -) (- - (7 0) -) (- (7 2) - -) (- - - (4 3)) ((7 3) - - -) (- - (7 0) -) (- (4 1) - -) (- - - (7 1)))

which can be "read" as a string, but will not "eval" the same as the function. For this statement, of course, what I need would be as simple as

(quote (((7 3...

but other examples are non-trivial. This one, for example, contains structs which print as vectors:

pair-of-jugglers
; -->
(#(struct:hand #(struct:position -0.35 2.0 1.0) #(struct:position -0.6 2.05 1.1) 1.832595714594046) #(struct:hand #(struct:position 0.35 2.0 1.0) #(struct:position 0.6 2.0500000000000003 1.1) 1.308996938995747) #(struct:hand #(struct:position 0.35 -2.0 1.0) #(struct:position 0.6 -2.05 1.1) -1.3089969389957472) #(struct:hand #(struct:position -0.35 -2.0 1.0) #(struct:position -0.6 -2.05 1.1) -1.8325957145940461))

I've thought of at least three possible solutions, none of which I like very much.

Help me out before I start having bad recursion dreams again.

Upvotes: 2

Views: 508

Answers (1)

Eli Barzilay
Eli Barzilay

Reputation: 29546

I'm not sure what you're trying to do. Specifically, trying to produce a file with eval-able code seems like a strange choice. In any case, creating a serialization of random values is a problem when you're dealing with structs -- since there could be different structs with the same name. Some points that might be relevant for you:

  • There is an undocumented library scheme/fasl that can read and write values in binary format (useful for large data).

  • There is also mzlib/pconvert -- a library that DrScheme uses to print values as expressions that can be evaluated (but this won't work on all kinds of data).

  • If you want to use structs that are very easy to write in a readable form, then you can use "prefab" structs.

(For more details it's probably best to ask on the mailing list.)

Upvotes: 4

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