Editing Keywords (section)

== forall ==

This is a GHC/Hugs extension, and as such is not portable Haskell 98/2010. 
It is only a reserved word within types. 

Type variables in a Haskell type expression are all assumed to be
universally quantified; there is no explicit syntax for universal
quantification, in standard Haskell 98/2010. For example, the type expression
<hask>a -> a</hask> denotes the type <hask>forall a. a ->a</hask>. 
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 forall extends as far to the right as
possible; for example, 
<haskell>
forall a. a -> a
</haskell>
means 
<haskell>
forall a. (a -> a)
</haskell>

GHC [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/poly_kinds.html#inferring-dependency-in-user-written-foralls introduces] a <hask>forall</hask> keyword, allowing explicit quantification, for example, to encode 
[http://downloads.haskell.org/~ghc/latest/docs/html/users_guide/exts/existential_quantification.html existential types]: 
<haskell>
data Foo = forall a. MkFoo a (a -> Bool)
         | Nil

MkFoo :: forall a. a -> (a -> Bool) -> Foo
Nil   :: Foo

[MkFoo 3 even, MkFoo 'c' isUpper] :: [Foo]
</haskell>