On Mon, Nov 22, 2010 at 2:29 PM, nicolas.o...@gmail.com
<nicolas.o...@gmail.com> wrote:
>> Type hints don't reduce flexibility AT ALL.
>>
>> user=> (defn t1 [x] (.length x))
>> #'user/t1
>> user=> (t1 "foo")
>> 3
>> user=> (t1 'foo)
>> #<CompilerException java.lang.IllegalArgumentException: No matching
>> field found: length for class clojure.lang.Symbol
>> (NO_SOURCE_FILE:141)>
>>
>> Since it calls a String method it doesn't work if you call it with a
>> non-String.
>
> It prevents duck-typing: t1 works on any object having a length method.
> t2 only works on String.
It prevents duck typing with direct interop. How common is that,
though? Most usually if there's a Java method with sufficiently
similar semantics as to be logically considered "the same method" on
multiple classes, there's a common interface or superclass they all
get it from and you can type hint to that. Duck typing internal to
clojure, such as calling seq on Strings and Collections, still works.
And you can make your own Clojure functions that duck type, e.g.
(defn close! [x]
(condp #(instance? %1 %2) x
Stream (.close ^Stream x)
JFrame (.dispose ^JFrame x)
ResultSet (.close ^ResultSet x)
URLConnection (.close ^URLConnection x)))
and then call (close! foo) on various resources to close them. More
clauses can be added as necessary. Or you can use defprotocol to make
a Closeable protocol that you can then extend for anything that has
.close, .dispose, or similar Java methods.
(The type hints in the above defn prevent you from losing the benefits
of type hinting by going through close. The caller of close! can type
hint various things and also close various things, and even close a
thing that might have different types at different times. You can also
(doall (map close! seq-of-resources)) and similarly.)
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