Re: [Haskell-cafe] showing a user defined type

(Apologies for my mutilation of ML syntax, I don't completely know the language) Consider the ML type int list, and this function to build one: broken_repeat :: int -> int list broken_repeat n = Cons(n, broken_repeat(n)) This function is recursive, and doesn't terminate; it tries to build an infinite list of ints and your computer runs out of heap and/or stack trying to evaluate it. But the "chain" type doesn't have this problem; you can see it as an int list that gets evaluated "on demand": repeat :: int -> chain repeat(n) = Link(n, repeat) always1 :: int -> chain always1(_) = Link(1, always1) chain_take :: int * chain -> int list chain_take (0,_) = Nil chain_take (n,Link(i,f)) = Cons(n, chain_take(n-1, f(i))) But, nothing in the "chain" type stops you from passing a different value to the function in the link: weird_take :: int * int * chain -> int list weird_take (0,_,_) = Nil weird_take (n,v,Link(i,f)) = Cons(i, weird_take(n-1,v,f(v))) Now, it's possible that chain_take returns the same list for two different "chain" inputs, but weird_take might return different lists depending on how f is implemented. For example: chain_take(5, repeat(1)) = [1,1,1,1,1] chain_take(5, always1(1) = [1,1,1,1,1] weird_take(5, 2, repeat(1)) = [1,2,2,2,2] weird_take(5, 2, always1(1)) = [1,1,1,1,1] One way to fix this is to embed the "state" of the chain in the closure itself. So, in ML, the type unit -> X is commonly called a "thunk"; it can be used to delay computation until later, until it's demanded, just like any lazy value in Haskell. f :: unit -> Int f () = 1 This f isn't very useful; it's basically the same as "1". But consider this type: datatype stream = Stream of (int * (unit -> stream)) stream1 () = Stream(1, stream1) stream_take :: int*stream -> int list stream_take(0,_) = Nil stream_take(n,Stream(i,f)) = Cons(i, stream_take(n-1, f())) Now there is no way to pass a different value like we did in weird_take; there's only (). The difference is that the state gets embedded in the closure for the thunk: stream_ints :: int -> (unit -> stream) stream_ints = fun n => fun () => Stream(n, stream_ints(n+1)) What you are doing here is encoding laziness; the Haskell version of this type:

data Stream = Stream !Int Stream -- !Int means the Int value is strict stream1 = Stream 1 stream1 stream_ints n = Stream n (stream_ints(n+1))

stream_take :: Int -> Stream -> [Int] stream_take 0 _ = [] stream_take n (Stream x xs) = x : stream_take (n-1) xs

No extra (\() -> ...) thunk is required, due to laziness :) -- ryan On Tue, May 19, 2009 at 4:25 PM, michael rice wrote:

Hi Ryan,

I'm afraid you've lost me. Maybe if you showed how this would be used in ML I would get the picture.

Michael

--- On Tue, 5/19/09, Ryan Ingram wrote:

From: Ryan Ingram Subject: Re: [Haskell-cafe] showing a user defined type To: "michael rice" Cc: "Brandon S. Allbery KF8NH" , haskell-cafe@haskell.org Date: Tuesday, May 19, 2009, 2:40 PM

On Tue, May 19, 2009 at 7:07 AM, michael rice wrote:

...

A little further along in "The Little MLer" the ints function is replaced by other functions like primes and fibs, which also return Links:

fun primes(n)   = if is_prime(n+1)  then Link(n+1,primes)  else primes(n+1)

fun fibs(n)(m)   = Link(n+m,fibs(m))

which are passed to chain_item:

fun chain_item(n,Link(i,f))   = if eq_int(n,1)   then i   else chain_item(n-1,f(i))

which can be called to request the nth (12th) prime number beginning at 1.

- chain_item(12,primes(1)); GC #0.0.0.1.3.61:   (1 ms) val it = 37 : int -

So I guess the answer to your question about whether the function is ever called with a different value may be, yes.

Actually, it's not calling it with another value; notice that chain_item calls f(i), with i coming directly from the chain. Consider this alternate definition: (I'm not sure the syntax is exactly right, but you get the idea)

datatype chain =   Link of (int * ( unit -> chain ))

fun intsFrom(n) = fun unit => (n, intsFrom (n+1)) fun ints(n) = intsFrom n ()

Now you *can't* call the function embedded in the link with another value.

fun chain_item(n,Link(i,f))   = if eq_int(n,1)   then i   else chain_item(n-1,f unit)

And this type for "chain" is almost the same as [Int] in Haskell, due to laziness.

  -- ryan