74 lines
1.4 KiB
Plaintext
74 lines
1.4 KiB
Plaintext
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(define-data-type list
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(null? null)
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(pair?
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(cons car cdr)
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(car type car set-car!)
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(cdr pair cdr set-cdr!)))
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=>
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(define null 0)
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cons etc. as before
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Type as before, except that LIST is defined to be a datatype
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that includes NULL and PAIRs. NULL and PAIRs are defined to
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be subtypes of LISTs. How do pairs and null get injected into
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lists?
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What about polymorphism? Ignore it for now.
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Note that setters have a problem with destructuring - they don't work.
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(if (bar? baz)
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...
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...)
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Need temporary type bindings, a la LET. Env and type env?
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Have to do shallow binding, the infer code doesn't use environments.
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Hacks like
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(if (bar? baz)
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(error ...))
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... baz is not a bar ...
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won't work.
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----------------
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New plan. Use functions for pattern matching, with a suitable macro.
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(define-data-type list delistify match-list
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(null? null)
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(pair?
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(cons car cdr)
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(car type car set-car!)
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(cdr pair cdr set-cdr!)))
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Defines null as 0 and pairs as before.
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NULL? and PAIR? are only available at load time. There is also a
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deconstructor DELISTIFY: [b list, [] -> a , [b, b list] -> a] -> a
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(define (map f l)
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(delistify l
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(lambda ()
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null)
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(lambda (car cdr)
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(cons (f car) (map f cdr)))))
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Or, with syntax,
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(define (map f l)
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(match-list l
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((null)
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null)
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((cons car cdr)
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(cons (f car) (map f cdr)))))
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This requires no change at all for the type checker. Hee Wack!
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