[Haskell-cafe] showing a user defined type
Ryan Ingram
ryani.spam at gmail.com
Wed May 20 14:21:44 EDT 2009
Actually, I was saying that "chain" already emulates laziness, just in
a somewhat unsafe way, as demonstrated by weird_take. In Haskell
you'd probably just write
> ints :: Int -> [Int]
> ints n = [n..]
and be done with it.
-- ryan
On Wed, May 20, 2009 at 9:41 AM, michael rice <nowgate at yahoo.com> wrote:
> OK, I think I understand. You were explaining how ML could be made to
> emulate Haskell laziness using streams, ala Scheme type delayed evaluation,
> so it's kind of like you were explaining a question I hadn't quite asked
> yet, which maybe explains my puzzlement, I hope.
>
> Also, though my understanding of both Haskell and ML syntax is still
> rudimentary, I did catch an error in your definition of chain_take: the
> first arg of the cons should be i rather than n.
>
> I'm still going through your code and may have further questions.
>
> Thanks for your input.
>
> Michael
>
>
> --- On Wed, 5/20/09, Ryan Ingram <ryani.spam at gmail.com> wrote:
>
> From: Ryan Ingram <ryani.spam at gmail.com>
> Subject: Re: [Haskell-cafe] showing a user defined type
> To: "michael rice" <nowgate at yahoo.com>
> Cc: "Brandon S. Allbery KF8NH" <allbery at ece.cmu.edu>,
> haskell-cafe at haskell.org
> Date: Wednesday, May 20, 2009, 4:12 AM
>
> (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 <nowgate at yahoo.com> 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 <ryani.spam at gmail.com> wrote:
>>
>> From: Ryan Ingram <ryani.spam at gmail.com>
>> Subject: Re: [Haskell-cafe] showing a user defined type
>> To: "michael rice" <nowgate at yahoo.com>
>> Cc: "Brandon S. Allbery KF8NH" <allbery at ece.cmu.edu>,
>> haskell-cafe at haskell.org
>> Date: Tuesday, May 19, 2009, 2:40 PM
>>
>> On Tue, May 19, 2009 at 7:07 AM, michael rice <nowgate at yahoo.com> 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
>>
>>
>
>
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