My question was on how to use coerce -- not whether this is worth doing or not. If we were discussing that, then I would say it's not 'just a leaf' -- it's ALL the leaves of the tree -- and for a tree of depth n, there are 2^n of them so depending on what 'n' is, it could potentially be something to worry about.

On Wed, Dec 2, 2020 at 6:00 PM Henning Thielemann <lemming@henning-thielemann.de> wrote:

On Wed, 2 Dec 2020, Brent Walker wrote:

> In the following code, function fmap does not compile because variable
> 'y' on line marked <***> has type (Expr a) where an (Expr b) is
> expected. The code can be fixed simply by returning (Val x) on the rhs
> of the function but then we are allocating a new object for something we
> could potentially reuse since (Val n) has the same runtime
> representation in (Expr a) and (Expr b) (it has no dependence on the
> type variable).

Is it worth the trouble? It is only a leaf of the tree.