Experimental: ToAlias(Reference)T doesn't evaluate for Maybes

[googleson78]

I got the following error today:

• Could not deduce: Exp.ToAliasReferenceT
                      (Exp.ToAliasT (SqlExpr (Maybe (Entity Thing))))
                    ~ SqlExpr (Maybe (Entity Thing))
    arising from a functional dependency between:
      constraint ‘Database.Esqueleto.Internal.Internal.SqlSelect
                    (Exp.ToAliasReferenceT
                       (Exp.ToAliasT (SqlExpr (Maybe (Entity Thing)))))
                    (Maybe (Entity Thing))’
        arising from a use of ‘select’

Is this an oversight in Experimental stuff, or am I trying to do something impossible/stupid?

Context is roughly as follows: I have a (nonempty) list of queries, all of which have a left outer join (with Thing being on the right side of the join), over which I'm doing a foldl1 Union

Basically this thing:

xs :: [B]
f :: B -> SqlQuery (SqlExpr (Entity C), SqlExpr (Maybe (Entity Thing))) -- this Thing is the same one from above

...
... <- select $ Experimental.from $ List.foldl1' Union $ map (SelectQuery . f) xs -- errors here
...

[parsonsmatt]

@belevy this seems like it should work, any idea what's going on?

[belevy]

Looks like an oversight in the type families. I might be able to take a look today.

EDIT: this only effects Entities. Values work fine. I will add this into my 3.3.3.3 patch which I guess is just now a bugfix roll-up.