[Haskell-cafe] Call for discussion: OverloadedLists extension

[Michael Snoyman]

On Tue, Sep 25, 2012 at 6:21 PM, Heinrich Apfelmus
<apfelmus at quantentunnel.de> wrote:
> Michael Snoyman wrote:
>>
>>
>> Note that I wasn't necessarily advocating such a pragma. And a lot of
>> my XML code actually *does* use two IsString instances at the same
>> time, e.g.:
>>
>>     Element ("img" :: Name) (singleton ("href" :: Name) ("foo.png" ::
>> Text)) [NodeComment ("No content inside an image" :: Text)]
>
>
> In this particular case, would it make sense to use smart constructors
> instead?
>
> The idea is that you can put the polymorphism in two places: either make the
> "output" polymorphic, or make the "input" polymorphic. The latter would
> correspond to a type
>
>    element :: (IsString name, IsString s, IsMap map)
>        => name -> map name s -> [Element]
>    element name map = Element (toName name) (toMap map)
>
> One benefit would be that the function will accept any list as a map, not
> just list literals.

Just to clarify: this would be a *replacement* for OverloadedStrings
usage, right? If used in conjunction with OverloadedStrings, we'd run
into the too-much-polymorphism issue you describe in your initial
email in this thread, since `element "foo'` would become `element
(fromString "foo")` which would become `Element ((toName . fromString)
"foo")`, and `toName . fromString` makes it ambiguous what the
intermediate data type is.

Assuming this is meant are a replacement, I see two downsides.
Firstly, this would work for construction, but not for deconstruction.
Currently, I can do something like:

handleList :: Element -> Element
handleList (Element "ul" _ _) = ...
handleList e = e

The other is that we've only solved one specific case by providing a
replacement function. In order to keep code just as terse as it is
now, we'd have to provide a whole slew of replacement functions. For
example, consider the code:

handleList (Element "ul" attrs _) = case Map.lookup "class" attrs of ....

If we get rid of OverloadedStrings, then we need to either provide a
replacement `lookup` function which performs the conversion from
String to Name, or change all lookup calls to explicitly perform that
lookup.

Michael