[Haskell-cafe] Coroutines

Mario Blazevic mblazevic at stilo.com
Thu Dec 18 13:45:10 EST 2008

Robin Green wrote:
> In my opinion, in Haskell, you don't need coroutines because you have
> lazy evaluation.
> You example below is simply an example of a heterogenous list being
> read. The simplest way to implement a heterogenous list in Haskell is
> to use a tuple. Or you could use the HList package.

	Not quite. The consumer, useSimple, is an example of a heterogenous 
list being read. The producer, simple, is an example of producing a 
heterogenous list, value by value, on demand. You don't get that from 
HList for free.

	The difference between coroutines and lazy evaluation is that in the 
latter the consumer has full control. Producer supplies all the thunks, 
consumer picks which thunk to evaluate.	Of course the producer could 
return a pair of first value and the rest of the computation, but then 
any control structure can be encoded using continuations. The question 
is one of notational convenience.

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