Flexible Contexts

Brief Explanation

In Haskell 98,

contexts of type signatures, newtype and data declarations consist of assertions of the form C v or C (v t₁ … t_n), where v is a type variable.
contexts of instance and class declarations consist of assertions of the form C v, where v is a type variable.

The proposal is that class arguments in contexts of type signatures and class declarations may be arbitrary types, e.g.

g :: (C [a], D (a -> b)) => [a] -> b

(Similar relaxation of instance declarations leads to UndecidableInstances. Contexts on newtype and data declarations are RemovalCandidates.)

References

Syntax of Class Assertions and Contexts in the Haskell 98 Report
Type classes: exploring the design space by Simon Peyton Jones, Mark Jones and Erik Meijer, Haskell Workshop 1997.
Type signatures in the GHC User's Guide

Tickets

#31	add Flexible Contexts

Variations

Hugs requires that type signatures be unambiguous (taking into account FunctionalDependencies). GHC requires only that type variables in the context be connected (by a chain of assertions) with type variables that occur in the head, so that it accepts
```
foo :: (C a b, C b c) => c -> Bool
```
even though such functions cannot be used without ambiguity.
GHC (but not Hugs) requires that the context contain at least one type variables that is universally quantified at that point. This is consistent with rejecting inferred types containing irreducible ground assertions (like Num Char, see below), which both GHC and Hugs do.

Context reduction

Haskell computes a type for each variable bound by let or where, and then generalizes this type. In Haskell 98, the allowed contexts are restricted, so contexts are reduced using instance declarations (and duplicate assertions and those implied by class contexts are removed) until either they are in the allowed form or no instance is applicable, in which case an error is reported. For example, in the following

module M where
class C a where c_method :: a -> Bool
foo xs = c_method (tail xs)

the context of the inferred type

foo :: C [a] => [a] -> Bool

is neither allowed nor reducible, so a missing instance C [a] is reported.

When restrictions on the form of contexts are removed, context reduction is forced only by

explicit signatures
the type of main
the MonomorphismRestriction
the no-escape restriction on ExistentialQuantification
the few remaining restrictions on contexts

The above example becomes legal; if a matching instance is in scope when context reduction is forced on uses of foo, they will also typecheck:

module Main where

import M

instance C [a] where c_method = null

main :: IO ()
main = print (foo "abc")

The exceptions in GHC and Hugs are:

contexts of derived instances are fully reduced, so the following instance is allowed only if the instance Eq Bar is in scope:
```
data Foo = K Bar deriving Eq
```
ground assertions are fully reduced, so that the following remains illegal:
```
foo x = x + 'a'
```

Delaying context reduction:

can leave contexts more complex (could interact with the Monomorphism Restriction)
delays (and sometimes avoids) type errors
sometimes avoids nontermination of context reduction for UndecidableInstances
is required by OverlappingInstances

Pros

useful with FlexibleInstances

Cons

complicated context reduction story
deferred error checking (FunctionalDependencies can ameliorate this)

FlexibleContexts