Proposal: ExplicitForall

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

Compiler support

GHC	mostly (ExplicitForAll; still allows forall as an expression id; allows superfluous vars to be quantified)
nhc98	none
Hugs	mostly (+98; still allows forall as an expression id; doesn't allow 0 tyvars to be quantified)
UHC	mostly (none; still allows forall as an expression id; allows superfluous vars to be quantified; doesn't allow 0 tyvars to be quantified)
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.

Report Delta

[incomplete]

Changes relative to H2010 report.

In  Section 2.4:

Replace:

 | foreign | if | import | in | infix | infixl

with:

 | forall | foreign | if | import | in | infix | infixl

---

In  Section 3:

Replace:

exp  →  infixexp :: [context =>] type  (expression type signature)

with:

exp  →  infixexp :: [forall tyvar1 … tyvark .] [context =>] type  (expression type signature, k ≥ 0)

---

In  Section 3.16:

Replace:

exp  →  exp :: [context =>] type

Expression type-signatures have the form e :: t, where e is an expression and t is a type (Section 4.1.2);
they are used to type an expression explicitly and may be used to resolve ambiguous typings due to
overloading (see Section 4.3.4). The value of the expression is just that of exp. As with normal type
signatures (see Section 4.4.1), the declared type may be more specific than the principal type derivable from
exp, but it is an error to give a type that is more general than, or not comparable to, the principal type.

with:

exp  →  exp :: [forall tyvar1 … tyvark .] [context =>] type  (k ≥ 0)

Expression type-signatures have the form e :: t, where e is an expression and t is a type (Section 4.1.2);
they are used to type an expression explicitly and may be used to resolve ambiguous typings due to
overloading (see Section 4.3.4). The value of the expression is just that of exp. As with normal type
signatures (see Section 4.4.1), the declared type may be more specific than the principal type derivable from
exp, but it is an error to give a type that is more general than, or not comparable to, the principal type.

The quantification [forall tyvar1 … tyvark .] is optional, but if given it must quantify exactly the set of
type variables used in [context =>] type (with one exception: that of the distinguished type variable in a
class declaration (Section 4.3.1), which must not be quantified).

---

In  Section 4:

Replace:

gendecl  →  vars :: [context =>] type  (type signature)

with:

gendecl  →  vars :: [forall tyvar1 … tyvark .] [context =>] type  (type signature, k ≥ 0)

---

In  Section 4.1.2:

Replace:

With one exception (that of the distinguished type variable in a class declaration (Section 4.3.1)), the type
variables in a Haskell type expression are all assumed to be universally quantified; there is no explicit syntax
for universal quantification [4]. For example, the type expression forall a . a -> a denotes the type ∀ a . a → a.
For clarity, however, we often write quantification explicitly when discussing the types of Haskell programs.
When we write an explicitly quantified type, the scope of the  extends as far to the right as possible; for
example, ∀ a . a → a means ∀ a . (a → a).

with:

The type variables in a type signature may be explicitly quantified with the forall keyword. All type
variables used in the type must be quantified, with one exception (that of the distinguished type
variable in a class declaration (Section 4.3.1), which must not be quantified). No additional type
variables may be quantified. The scope of the quantifier extends as far to the right as possible.
For example, the type expression a -> a denotes the type ∀ a . a → a. Quantifying the type variables is
optional. For example, the type expression a -> a also denotes the type ∀ a . a → a. For convenience, we
write forall vs . t even if there is no forall vs ..

---

In  Section 4.3.1:

Replace:

for which there is an explicit type signature vi :: cxi => ti in cdecls.

with:

for which there is an explicit type signature vi :: forall vs . cxi => ti in cdecls.

---

In  Section 4.1.4:

Replace:

(Eq a, Show a, Eq b) => [a] -> [b] -> String

with:

forall a b . (Eq a, Show a, Eq b) => [a] -> [b] -> String

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.