[Haskell-cafe] Byte Histogram

Richard O'Keefe ok at cs.otago.ac.nz
Mon Feb 7 02:46:30 CET 2011

On 7/02/2011, at 8:41 AM, Andrew Coppin wrote:

> On 06/02/2011 09:13 AM, Roel van Dijk wrote:
>> Haskell en Clean are very much alike.
> From what I could determine from a basic Clean introduction, Clean is very *unlike* Haskell, having a far more verbose and low-level syntax. (E.g., the compiler can't even determine whether a binding is recursive or not for itself. You have to say that manually.)

I have no idea what you are talking about here.
Clean is _very_ Haskell-like, including typeclasses.

Here's the first few lines of code from a Clean file I wrote in 1998.

// This is a 'data' declaration.
  :: ArrTree a
   = ArrEmpty
   | ArrLeaf a
   | ArrNode a (ArrTree a) (ArrTree a)

// The parentheses were not necessary
  empty :: (ArrTree a)
  empty = ArrEmpty

  asize :: (ArrTree a) -> Int
  asize (ArrEmpty)      = 0
  asize (ArrLeaf _)     = 1
  asize (ArrNode _ l r) = 1 + asize l + asize r

// In Haskell it would be Int -> (ArrTree a) -> Bool.
// Leaving the first arrow out means that both arguments
// must be present in each rule.
// 'if' is a function.
  known :: Int (ArrTree a) -> Bool
  known i ArrEmpty        = False
  known i (ArrLeaf _)     = i == 1
  known i (ArrNode x l r) = i == 1 || known (i/2) (if (i mod 2 == 0) l r)

  fetch :: Int (ArrTree a) -> a
  fetch i (ArrLeaf x)     | i == 1 = x
  fetch i (ArrNode x l r) | i == 1 = x
                          | i mod 2 == 0 = fetch (i/2) l
                          | otherwise    = fetch (i/2) r

As for the compiler being unable to determine whether a binding is recursive,
I cannot find any such restriction in the Clean 2.1.1 manual and don't remember
one in Clean 1.  Here's an example straight out of the manual:

primes :: [Int]
primes = sieve [2..]
    sieve :: [Int] -> [Int]
    sieve [pr:r] = [pr:sieve (filter pr r)]

    filter :: Int [Int] -> [Int]
    filter pr [n:r]
      | n mod pr == 0 = filter pr r
      | otherwise     = [n:filter pr r]

Clean uses [head : tail] where Haskell uses (head : tail).
sieve and filter are both recursive (local) bindings, and the
compiler manages just FINE.

> It seems a very unecessarily complicated and messy language - which makes the name rather ironic.

It would be if true.  There _are_ complexities in Clean, just as there are in
Haskell.  For the most part, they are the same complexities (laziness, type classes,
type inference, generic programming).

> As I say, I thought the main difference is that Clean is strict


> (which is why you can get good performance). Uniqueness typing is an interesting idea, that looks like it might be useful for more than mere I/O.

It has been much used for arrays and records...

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