> in GHC we quite often we want to get the entry or the last node of a block.
 > Currently that can take up to linear time in the length of a block, right?
 > Are there any plans to improve it, perhaps by changing the representation
 > of a block?

Milan,

The current representation can be either 'front-biased' or
'back-biased' to make one of those operations constant time.
The previous representation was always front-biased and we never noted
any performance problems.

If, and only if, you find a situation where you have measured a
significant performance problem, I suggest that you wrap the existing
block in a new block type that caches the information you need.
The representation Graph' is polymorphic precisely to enable this sort
of trickery; you'll see a different application in the dataflow
module, where a fact is stored with every block.