A RULE could work. Assuming fromListLike is inlined, a rule can detect when (fromList . toList :: a -> a) and then replace it with id. edward Excerpts from David Fox's message of 2016-06-19 16:01:01 -0700:
Suppose you have a class ListLike (which you do have, actually) and it has methods for operations on types that are like lists, and in particular it has a method fromListLike which converts any ListLike value to any other:
fromListLike :: ListLike full' item => full' -> full
the default implementation of this is simply
fromListLike = fromList . toList
but this means that if you happen to apply fromListLike to a value which is already the desired type, two unnecessary and possibly expensive conversions take place. My question is, how can one write code that implements fromListLike in such a way that when the two type parameters are the same type it just uses
fromListLike = id
I've thought about using a class Convert a b class and writing an instance for every pair of ListLike instances. I haven't convinced myself this would work, and if it does it means that adding a new instance involves writing a bunch of Convert instances. Maybe one could somehow have a default implementation (fromList . toList) but then you run into overlapping instances. Anyone have better ideas about any of this? Hopefully I'm missing something dead simple...