Hi, I'm playing around with Type Families so I decided to implement a simple fixed-length Vect of Integers. Here is my straightforward implementation (ghc 7.8.3): https://gist.github.com/anonymous/d838e68ce6a02412859f As you can see, I'm trying to implement a reverse function for my vectors which guarantees that the output vector is the same size as the input one. I tried to express this constraint at the type level. The problem is that I can't have ghc to type check the reverse function in the non-trivial case: _________________ Could not deduce (Plus n1 (S m) ~ S (Plus n1 m)) from the context (n ~ S n1) bound by a pattern with constructor CV :: forall n. Int -> Vect n -> Vect (S n), in an equation for ‘vecReverse’ at vect3.hs:30:13-18 Expected type: Vect (Plus n m) Actual type: Vect (Plus n1 (S m)) _________________ Iit has to do with the fact that the type checker can't deduce that: Plus n1 (S m) ~ S (Plus n1 m) ~ Plus (S n1) m ~Plus n m I tried to insert the following instance to the family: Plus n (S m) = S (Plus n m) but to no avail. Any clue? Thanks. C. -- GPG Key: 4096R/C17E53C6 2010-02-22 Fingerprint = 4575 4FB5 DC8E 7641 D3D8 8EBE DF59 B4E9 C17E 53C6