Hi,
Suppose we have some data structure that uses HOAS; typically, a DSL
with explicit sharing. For example:
> data Expr = One | Add Expr Expr | Let Expr (Expr -> Expr)
When I use such a data structure, I find myself writing expressions such
as
> Let foo $ \a ->
> Let bar $ \b ->
> Add a b
It seems to me that there should be a monad here somewhere, so that I
can write this approximately like
do a <- foo
b <- bar
return (Add a b)
or something along those lines. It is however not at all clear to me
what this monad should look like. I have experimented with adding a type
parameter to Expr, something like
> data Expr a = One
> | Add (Expr a) (Expr a)
> | Let (Expr a) (Expr a -> Expr a)
> | Place a
which has the additional benefit of supporting catamorphisms; I have
also experimented with defining various variations on
> Let a b = Let a (a -> b)
and making Let an instance of Monad rather than Expr; but none of my
experiments wered satisfactory.
Am I missing something obvious, or is there a good reason this cannot be
done?
Thanks,
Edsko
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