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What static typing makes difficult

james anderson james.anderson at setf.de
Sat Oct 25 18:49:08 EDT 2003 
"Brian McNamara!" wrote:
> 
> james.anderson at setf.de once said:
> ...
> >
> >? would haskell suggest to handle the attribute-based specialization in a
> >manner similar to the original post, or does it offer some other mechanism?
> 
> There are two ways to go.  If it's reasonable to know statically about
> this attribute, then you could encode it in the type system (e.g. a type
> like DOMWithDocumentElement).  My hunch (based on my very limited
> knowledge of the domain) is that this is a dynamic attribute.  So
> instead you'd 'fail' if the attribute didn't match at run-time.  Note
> that my Haskell code returned "Maybe XMLRep"s, so the corresponding code
> would look something like
> 
>    -- has type Maybe XMLRep
>    if (hasAttr domObj "documentElement")
>    then Just ...  -- do the conversion of the domObj
>    else Nothing
> 
> The same strategy would be used to cope with other 'dynamic failures'
> in the other branches, such as a malformed XML string, or a
> non-existent file when converting from a filename.

one favor which clos brings to the party is a fairly direct means to express
type constraints with arbitrary valence. generic functions turn the original
__init__ sort of "inside-out" and suggest an expression more like:

(defclass XML_Objectify () ())

(defgeneric __init__ (what from)
  (:method ((instance XML_Objectify) (source string))
           (case (char source 0)
             (#\< (with-input-from-string (stream source)
                    (__init__ instance stream)))
             (t (__init__ instance (intern-uri source)))))
  (:method ((instance XML_Objectify) (source http-uri))
           (with-open-uri (stream source :method :get)
             (__init__ instance stream)))
  (:method ((instance XML_Objectify) (source pathname))
           (with-open-file (stream source :direction :input)
             (__init__ instance stream)))
  (:method ((instance XML_Objectify) (source stream))
           (__init__ instance (parse-document source)))
  (:method ((instance XML_Objectify) (source xml-document))
           (dolist (element (children (root source)))
             (setf (slot-value instance (name-symbol (name element)))
                   (read-from-string (value element))))))

if one needed additional specialization, one could add additional parameters,
as in

(defgeneric __init__ (what from how) ...)

from some perspectives this is "easy". from others perhaps not. 
...
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