[racket] sharing an expensive computation required in procedure and its contract?

Though it is an off-label use of contracts, it can be done. I made something I call "coercion contracts" that mutate & validate the input (or output) argument as part of the contract:

(define/contract (function-that-wants-something-stringlike x)
    (coerce/string? . -> . coerce/boolean?)
    (do-things-to x))

Source code is here:
https://github.com/mbutterick/sugar/blob/master/coerce/contract.rkt

Or do `raco pkg install sugar` and (require sugar/coerce/contract) 


Please note: I am setting a terrible example.




>> I feel like this has come up before, but I can't seem to find the
>> discussion in the archive. I have a procedure that needs to perform a
>> fairly expensive computation which is needed both for checking the
>> inputs and for producing output. I'd like to avoid computing it twice,
>> and I'd also like to use contracts to check the inputs. Is there any
>> reasonable way to do this?
> 
> 
> No. The contract system is designed so that the contractual 
> computations are in principle independent from the 'mutator'
> computations. 
> 
> 
>> The code is like this:
>> 
>> (define (do-stuff x y z)
>> (define important-value (expensive x y z))
>> ... do stuff with important-value ...)
>> 
>> (provide/contract
>> [do-stuff (->i [x ...] [y ...] [z (x y)
>> contract-that-depends-on-important-value]) [result ...])])
>> 
>> 
>> I could memoize the expensive computation, but that would require
>> something like an LRU cache to avoid memory problems, and I'd rather
>> not have to worry about that.
>> 
>> Alternatively, I could shift things around so that the contract is on
>> the procedure that generates important-value, but then blame reporting
>> would be incorrect.
>> 
>> Any ideas?
> 
> 
> Define a syntactic abstraction that spans contracts and 'mutators'. 
> 
> -- Matthias
> 
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