Parametric Polymorphism 
Section goal = short, enables reader to read code (ParseP) with ∀ and use libraries (ST) without horror. Question Talk:Haskell/The_Curry-Howard_isomorphism#Polymorphic types would be solved by this section.
Link to the following paper: Luca Cardelli: On Understanding Types, Data Abstraction, and Polymorphism.
forall a 
As you may know, a polymorphic function is a function that works for many different types. For instance,
length :: [a] -> Int
can calculate the length of any list, be it a string
String = [Char] or a list of integers
[Int]. The type variable
a indicates that
length accepts any element type. Other examples of polymorphic functions are
fst :: (a, b) -> a snd :: (a, b) -> b map :: (a -> b) -> [a] -> [b]
Type variables always begin in lowercase whereas concrete types like
String always start with an uppercase letter, that's how we can tell them apart.
There is a more explicit way to indicate that
a can be any type
length :: forall a. [a] -> Int
In other words, "for all types
a, the function
length takes a list of elements of type
a and returns an integer". You should think of the old signature as an abbreviation for the new one with the
forall. That is, the compiler will internally insert any missing
forall for you. Another example: the types signature for
fst is really a shorthand for
fst :: forall a. forall b. (a,b) -> a
fst :: forall a b. (a,b) -> a
Similarly, the type of
map is really
map :: forall a b. (a -> b) -> [a] -> [b]
The idea that something is applicable to every type or holds for everything is called universal quantification. In mathematical logic, the symbol ∀ (an upside-down A, read as "forall") is commonly used for that, it is called the universal quantifier.
Higher rank types 
forall, it now becomes possible to write functions that expect polymorphic arguments, like for instance
foo :: (forall a. a -> a) -> (Char,Bool) foo f = (f 'c', f True)
f is a polymorphic function, it can be applied to anything. In particular,
foo can apply it to both the character
'c' and the boolean
It is not possible to write a function like
foo in Haskell98, the type checker will complain that
f may only be applied to values of either the type
Char or the type
Bool and reject the definition. The closest we could come to the type signature of
foo would be
bar :: (a -> a) -> (Char, Bool)
which is the same as
bar :: forall a. ((a -> a) -> (Char, Bool))
But this is very different from
forall at the outermost level means that
bar promises to work with any argument
f as long as
f has the shape
a -> a for some type
a unknown to
bar. Contrast this with
foo, where it's the argument
f who promises to be of shape
a -> a for all types
a at the same time , and it's
foo who makes use of that promise by choosing both
a = Char and
a = Bool.
Concerning nomenclature, simple polymorphic functions like
bar are said to have a rank-1 type while the type
foo is classified as rank-2 type. In general, a rank-n type is a function that has at least one rank-(n-1) argument but no arguments of even higher rank.
The theoretical basis for higher rank types is System F, also known as the second-order lambda calculus. We will detail it in the section System F in order to better understand the meaning of
forall and its placement like in
Haskell98 is based on the Hindley-Milner type system, which is a restriction of System F and does not support
forall and rank-2 types or types of even higher rank. You have to enable the
RankNTypes language extension to make use of the full power of System F.
But of course, there is a good reason that Haskell98 does not support higher rank types: type inference for the full System F is undecidable, the programmer would have to write down all type signatures himself. Thus, the early versions of Haskell have adopted the Hindley-Milner type system which only offers simple polymorphic function but enables complete type inference in return. Recent advances in research have reduced the burden of writing type signatures and made rank-n types practical in current Haskell compilers.
For the practical Haskell programmer, the ST monad is probably the first example of a rank-2 type in the wild. Similar to the IO monad, it offers mutable references
newSTRef :: a -> ST s (STRef s a) readSTRef :: STRef s a -> ST s a writeSTRef :: STRef s a -> a -> ST s ()
and mutable arrays. The type variable
s represents the state that is being manipulated. But unlike IO, these stateful computations can be used in pure code. In particular, the function
runST :: (forall s. ST s a) -> a
sets up the initial state, runs the computation, discards the state and returns the result. As you can see, it has a rank-2 type. Why?
The point is that mutable references should be local to one
runST. For instance,
v = runST (newSTRef "abc") foo = runST (readSTRef v)
is wrong because a mutable reference created in the context of one
runST is used again in a second
runST. In other words, the result type
(forall s. ST s a) -> a may not be a reference like
STRef s String in the case of
v. But the rank-2 type guarantees exactly that! Because the argument must be polymorphic in
s, it has to return one and the same type
a for all states
s; the result
a may not depend on the state. Thus, the unwanted code snippet above contains a type error and the compiler will reject it.
- predicative = type variables instantiated to monotypes. impredicative = also polytypes. Example:
length [id :: forall a . a -> a]or
Just (id :: forall a. a -> a). Subtly different from higher-rank.
- relation of polymorphic types by their generality, i.e. `isInstanceOf`.
- haskell-cafe: RankNTypes short explanation.
System F 
Section goal = a little bit lambda calculus foundation to prevent brain damage from implicit type parameter passing.
- System F = Basis for all this ∀-stuff.
- Explicit type applications i.e.
map Int (+1) [1,2,3]. ∀ similar to the function arrow ->.
- Terms depend on types. Big Λ for type arguments, small λ for value arguments.
Section goal = enable reader to judge whether to use data structures with ∀ in his own code.
- Church numerals, Encoding of arbitrary recursive types (positivity conditions):
&forall x. (F x -> x) -> x
- Continuations, Pattern-matching:
I.e. ∀ can be put to good use for implementing data types in Haskell.
Other forms of Polymorphism 
Section goal = contrast polymorphism in OOP and stuff. how type classes fit in.
- ad-hoc polymorphism = different behavior depending on type s. => Haskell type classes.
- parametric polymorphism = ignorant of the type actually used. => ∀
Free Theorems 
Secion goal = enable reader to come up with free theorems. no need to prove them, intuition is enough.
- free theorems for parametric polymorphism.
- Note that the keyword
forallis not part of the Haskell 98 standard, but any of the language extensions
RankNTypeswill enable it in the compiler. A future Haskell standard will incorporate one of these.
UnicodeSyntaxextension allows you to use the symbol ∀ instead of the
forallkeyword in your Haskell source code.
- Or enable just
Rank2Typesif you only want to use rank-2 types
- John Launchbury; Simon Peyton Jones (1994-??-??). Lazy functional state threads. ACM Press". pp. 24-35. http://www.dcs.gla.ac.uk/fp/papers/lazy-functional-state-threads.ps.Z.
See also 
- Luca Cardelli. On Understanding Types, Data Abstraction, and Polymorphism.
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