[Haskell-cafe] How to avoid expensive and unnecessary type conversions?

[David Fox]

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...

[Edward Z. Yang]

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:

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[David Fox]

Wow, I had no idea that existed!