Equality constraint type mismatch

[Simon Peyton-Jones]

| type family T a :: *
| my_id :: f a -> f a; my_id = id
| x :: T a -> T a; x = my_id
| 
| 
| IMHO, this was not clear from the documentation or from the error
| message (It certainly _looks_ like "T a" should match "f a"...).
| Thanks for the explanation.

Did you read this?
	http://haskell.org/haskellwiki/GHC/Type_families#Injectivity.2C_type_inference.2C_and_ambiguity

Please edit it to improve the clarity, or add a whole new sub-section if you think that would be clearer.  It's a wiki.

Simon