[Haskell-cafe] Eta Reduction

Edward Kmett ekmett at gmail.com
Wed Apr 2 13:51:46 UTC 2014

Consider something like

data Foo a = Foo !Int !a

The data constructor Foo needs to be strict in its arguments, so it'd need Seq
Int and Seq a. Seq Int would be resolved by the environment, but Seq a would
need to come from somewhere.

data Seq a => Foo a = Foo !Int !a

gives you

Foo :: Seq a => Int -> a -> Foo a

so it is available when evaluating the constructor.

We're not storing the instance as a slot in the constructor, so this isn't.

data Foo a where
   Foo :: Seq a => Int -> a -> Foo a

or equivalently

data Foo a = Seq a => Foo !Int !a

This is where the data type contexts came from. They were originally
motivated by the needs of the no-longer existent Seq class, and then
subverted to make Complex less useful than it otherwise could be. ;)


On Wed, Apr 2, 2014 at 3:08 AM, Tom Ellis <
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:

> On Tue, Apr 01, 2014 at 05:32:45PM -0400, Edward Kmett wrote:
> > Unfortunately the old class based solution also carries other baggage,
> > like the old data type contexts being needed in the language for bang
> > patterns.  :(
> Can you explain why that is so?  I don't understand.
> Tom
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