[Haskell-cafe] Language semantics

[Andrew Coppin]

Dan Mead wrote:
> Andrew: Try using catchalls in your guards
>
>
> pattern1
> | guard1 =
> | guard2 =
> | otherwise =
>
> This makes it much easier to use pattern guards.
> "otherwise" is a reserved word used for this stuff in ghc.

Yeah, it's good practice to include an explicit otherwise clause - but 
in this specific instance, I'm trying to do tricky stuff where one rule 
falls through to another if some condition doesn't hold - but that 
condition is only expressible as a function.

Well, let me show you the code - somebody will probably recognise this 
stuff...

convert1 :: Expression -> Expression
convert1 S = S
convert1 K = K
convert1 I = I
convert1 (Var v) = Var v
convert1 (x :@: y) = (convert1 x) :@: (convert1 y)
convert1 (Lam n e)
  | n `not_in` e = K :@: (convert1 e)
convert1 (Lam n (Var v))
  | n == v = I
  | otherwise = K :@: (convert1 (Var v))
convert1 (Lam n (x :@: y))
  | y `is_var` n && n `not_in` x = convert1 x
  | otherwise                    = S :@: (convert1 (Lam n x)) :@: 
(convert1 (Lam n y))
convert1 (Lam n (Lam m e))
  | n `not_in` e = K :@: (convert1 (Lam m e))
  | otherwise    = convert1 (Lam n (convert1 (Lam m e)))