[Haskell-cafe] Are there standard idioms for lazy, pure error handling?

[Ketil Malde]

Malcolm Wallace <malcolm.wallace at cs.york.ac.uk> writes:

> Errm, you mean:         4 `Then` 5 `Then` 1 `Then` Finally "success!"

Yes, sorry, and thanks.  I guess I should learn to check with ghci
before posting... How about this for a nicer syntax?

  infixr 8 :+
  infixr 8 +:

  data TList a e = a :+ (TList a e)
              | Return e deriving Show
              
  x +: y = x :+ (Return y)

  *Main> 2 :+ 4 +: "success"
  2 :+ (4 :+ Return "success")

I like the generic terminal value, it allows things like:

  *Main> let count = go 0 where go i (x:xs) = x :+ go (i+1) xs; go i [] = Return i
  *Main> :t count
  count :: [t] -> TList t Integer
  *Main> count [1..5]
  1 :+ (2 :+ (3 :+ (4 :+ (5 :+ Return 5))))

(But perhaps these things can be done more elegantly using State or
similar?) 

-k
-- 
If I haven't seen further, it is by standing in the footprints of giants