[Haskell-cafe] List spine traversal

[Matthias Görgens]

> As a side note, (allowing seq and unsafePerformIO if necessary) is it
> possible to implement a map that preserves cycles (instead of
> transparently replacing them with infinite copies?  Not horribly
> useful, but would be quite cute.

Baltasar Trancon y Widemann gave a talk on a generalized version of
this problem at HaL4.  Short answer: The problem is tractable in
theory, but you need heavy math.

Matthias.