[jira] Created: (BOO-246) Implicit duck typing option

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  A new issue has been created in JIRA.

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View the issue:
  http://jira.codehaus.org/browse/BOO-246

Here is an overview of the issue:
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        Key: BOO-246
    Summary: Implicit duck typing option
       Type: New Feature

     Status: Unassigned
   Priority: Major

 Original Estimate: Unknown
 Time Spent: Unknown
  Remaining: Unknown

    Project: Boo
 Components: 
             Compiler

   Assignee: 
   Reporter: Doug Holton

    Created: Thu, 3 Feb 2005 3:58 PM
    Updated: Thu, 3 Feb 2005 3:58 PM

Description:
This idea was mentioned in a few mailing list posts:
http://archive.boo.codehaus.org/user/messages/530
http://archive.boo.codehaus.org/user/messages/558
http://archive.boo.codehaus.org/user/messages/559
(Rodrigo's response is not in the archives)

When a type in boo cannot be inferred, it resolves to "object".  For example, 
this code will not work because "ToUpper()" is a method of type string, not 
type object:

l = [1,"asdf",3]
for item in l:  //item's type is just "object"
        if item isa string:
                print item.ToUpper()
        else:
                print item

Currently you can cast to a string, which is fastest:
print (item as string).ToUpper()

or you can cast the item to a "duck" type so that the method resolution doesn't 
happen until runtime (this is slower):

for item as duck in l:
       item.ToUpper() //now works if the item is really a string


We can create a compiler option to implicitly treat unknown types as type 
"duck" instead of type "object", so no "as duck" is necessary.  This might be 
helpful for lighter scripting tasks where speed is not so important.

For example to make the above code sample work, in 
src/Boo.Lang.Compiler/TypeSystem/TypeSystemServices.cs, change the end of the 
GetEnumeratorItemType method (at line 424) from this:
return ObjectType;
to this:
return DuckType;

To make implicit duck typing optional, however, we would need to add the option 
to the Boo compiler (or else implement it as a compiler service like Rodrigo 
has discussed for other future options).


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