FlexibleContexts is under specified

I've been reading the documentation on FlexibleContexts. It's pretty confusing.

Structure

The documentation is scattered around 3 different sections:

• 9.8.1.2. The superclasses of a class declaration (this is where the link from the flag reference takes you).
• 9.8.3.3. Relaxed rules for instance contexts
• 9.15.2. The context of a type signature

Maybe it's ok to separate it like this, but it took me a while to figure out that there //are// actually three different sections. A simple solution would be to mention that there are 3 different sections, in each section (it already does so in the last section).

Specification

The -XFlexibleContexts flag lifts the Haskell 98 restriction that the type-class constraints in a type signature must have the form (class type-variable) or (class (type-variable type1 type2 ... typen)).

That's all fine and well, but hardly teaches me anything.

I would like to know which type-class constraints //are// allowed with FlexibleContexts. Or in other words: what is a definition of a "type-class constraint" with FlexibleContexts?

This is the old grammar:

context 	→ 	class
	| 	( class1 , … , classn ) 	    (n ≥ 0)
class 	→ 	qtycls tyvar
	| 	qtycls ( tyvar atype1 … atypen ) 	    (n ≥ 1)

What is the new one?

Examples

• These are the example mentioned in the section on the context of a class declaration (9.8.1.2):

class Functor (m k) => FiniteMap m k where

class (Monad m, Monad (t m)) => Transform t m where
  lift :: m a -> (t m) a

Problem is, they neither require FlexibleContexts, nor do they compile with FlexibleContexts!

Those examples require MultiParamTypeClasses. MultiParamTypeClasses doesn't enable FlexibleContexts automatically.

• The section on instance contexts doesn't have any examples.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
Test case
Differential revisions
BlockedBy
Related	#12010 (closed)
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Operating system
Architecture