Contexts in @data@ and @type@

[Simon L Peyton Jones <simonpj@cs.glasgow.ac.uk>]

To: haskell@cs.glasgow.ac.uk
Subject: Contexts in @data@ and @type@
Date: Fri, 01 Mar 91 17:15:06 +0000
From: Simon L Peyton Jones <simonpj@cs.glasgow.ac.uk>
Sender: haskell-request@cs.glasgow.ac.uk


Nobody has been able to give a satisfactory account of what
contexts in data and type declarations ACTUALLY MEAN.  (see for
example Sections 4.1.3, 4.3.2, etc).

I propose the following simplifications.

PROPOSAL
~~~~~~~~

1. Contexts in @type@ declarations should vanish from the language.
Nobody has been able to explain what they mean.

2. The declaration

	data (Eq a, Eq b) => Foo a b = C1 b a | C2 b Int | C3 

introduces the (perfectly ordinary) type constructor Foo, and also introduces
typings for C1, C2, C3 as follows:

	C1 :: forall a,b . (Eq a, Eq b) => b -> a -> Foo a b
	C2 :: forall a,b .  Eq b        => b -> Int -> Foo a b
	C3 :: forall a,b .                 Foo a b

AND THAT IS THE *ONLY* WAY IN WHICH Foo IS SPECIAL.

In general, each constructor gets a context which is a subset of that
given in the @data@ decl, containing all the constraints on the free
type variables of the constructor signature, and no others.
(I was persuaded of this minor complication by John's comments, and
by the fact that many more cases of ambiguity are likely to arise 
otherwise.)

End of proposal
~~~~~~~~~~~~~~~