[Haskell-cafe] `zip` doesn't work with infinite `Seq`s

[Chris Wong]

> Data.Sequence provides a general-purpose *finite* sequence. There is
> no such thing as an infinite Seq!
>
> In fact, you'll find that while
>
>     head $ repeat 'a'
>
> results in 'a',
>
>     Seq.head . Seq.fromList $ repeat 'a'
>
> never returns.

To add to my previous comment: the key feature of Seq is constant-time
access to both ends of the sequence. It does this by caching the the
first and last few elements in the constructor.

Given these constraints, the behavior you observe makes sense. To
construct a Seq (as the call to fromList does), we must find the last
element in the list so that we can cache it. But an infinite list
doesn't have a last element (by definition). So fromList never
terminates.

I don't think there's a way to allow infinite sequences while also
having efficient access to both ends. The Halting Problem probably
comes into it somewhere. The solution you gave is likely the best one.

Chris