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...