Proposal: ExplicitForall

Ticket	#133
Dependencies
Related	Rank2Types, RankNTypes, LiberalTypeSynonyms, PolymorphicComponents, ScopedTypeVariables

Compiler support

GHC	[ full (ExplicitForAll?) ]
nhc98	[ none ]
Hugs	[ full (+98) ]
UHC	[ full (none) ]
JHC	[ none ]
LHC	[ none ]

Description

ExplicitForAll? enables the use of the keyword 'forall' to make a type explicitly polymorphic. Syntactically, it would mean the following change to Haskell 98:

• forall becomes a reserved word.
• . (dot) becomes a special (not reserved) operator.
• The following syntactic rule changes:

type → forall tyvars . type

| context => type

| ftype

ftype → btype -> type

| btype

gendecl → vars :: type

It does not allow the use of explicitly polymorphic types in any way not already allowed by Haskell 98 for implicitly polymorphic types.

References

 http://www.haskell.org/pipermail/haskell-prime/2009-June/002786.html

Pros

• Small and simple syntactic extension.
• Simplifies the later inclusion of semantic extensions that depend on it, e.g. Rank2Types.
• Easy to implement in tools that don't yet support the semantic extensions.
• The Report already mentions types using the explicit forall-quantified form, so only the grammar changes above are needed.

Cons

• A small and incremental extension with little value of its own, only serving as a stepping stone for the various semantic extensions.