[Haskell-cafe] Question on Coerce

[David Feuer]

You could use unsafeCoerce, but I don't think it's worth the risk. With the
plain code, GHC will *try* to recover sharing in cases like this late in
compilation, after the types are erased.

On Wed, Dec 2, 2020, 10:12 AM Brent Walker <brenthwalker at gmail.com> 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).
>
> I was reading about coerce (Data.Coerce) recently and thought this could
> be a place it could be used but simply replacing the 'y' on the rhs with
> 'coerce y' does not compile (error message below).
>
> Is it possible to use coerce in this context?
>
> Thanks for any help,
> Brent
>
> =======================================
> data Expr a = Var a | Val Int | Add (Expr a) (Expr a)
>   deriving Show
>
> instance Functor Expr where
>   fmap :: (a -> b) -> Expr a -> Expr b
>   fmap f (Var a) = Var (f a)
>   fmap f (Add e0 e1) = Add (fmap f e0) (fmap f e1)
>   fmap _ y@(Val x) = y   -- <***>
>
> =======================================
> /dev/proj/src/Ex12.hs:69:22: error:
>     • Couldn't match representation of type ‘a’ with that of ‘b’
>         arising from a use of ‘coerce’
>       ‘a’ is a rigid type variable bound by
>         the type signature for:
>           fmap :: forall a b. (a -> b) -> Expr a -> Expr b
>         at /dev/proj/src/Ex12.hs:66:11-38
>       ‘b’ is a rigid type variable bound by
>         the type signature for:
>           fmap :: forall a b. (a -> b) -> Expr a -> Expr b
>         at /dev/proj/src/Ex12.hs:66:11-38
>     • In the expression: coerce y
>       In an equation for ‘fmap’: fmap _ y@(Val _) = coerce y
>       In the instance declaration for ‘Functor Expr’
>     • Relevant bindings include
>         y :: Expr a (bound at /dev/proj/src/Ex12.hs:69:10)
>         fmap :: (a -> b) -> Expr a -> Expr b
>           (bound at /dev/proj/src/Ex12.hs:67:3)
>    |
> 69 |   fmap _ y@(Val _) = coerce y
>    |                      ^^^^^^^^
> =======================================
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