Re: Proposal: Make intersperse lazier

On Thursday 16 September 2010 17:08:08, Duncan Coutts wrote:

On 16 September 2010 15:39, Daniel Fischer wrote:

...

The current implementation of Data.List.intersperse causes a space leak under certain not uncommon circumstances. Trac ticket: http://hackage.haskell.org/trac/ghc/ticket/4282 The proposed implementation,

changes the semantics from

intersperse (x : _|_) = _|_

to

intersperse (x : _|_) = x : _|_

You mean:

intersperse sep (x : _|_) = x : _|_

Oops, of course.

We would also need to change intercalate to match:

intercalate sep (x : _|_) = x ++ _|_

Note that the current definition of intercalate in the base package will not need to change because it is defined in terms of intersperse, but its strictness specification does change.

Assuming the proposal is updated with the corresponding change to intercalate,

You mean, also mention the consequences for intercalate explicitly? Or something more/else?

then I support the proposal. It is in the spirit of the Haskell98 List module for list functions to be as lazy as possible (except there are good reasons to be stricter, e.g. as in splitAt). Incidentally, the lazier version is also slightly more efficient.

Duncan