On Sun, 22 Apr 2018, Tom Ellis wrote:
Unless I am much mistaken `Cols s a` is just `a`[1,2]. That explains why the type is ambiguous. There's nothing that fixes `s`.
I also thought that, but if I substitute `Cols s a` by `a`, then the compilation error is: ``` • Couldn't match type ‘Res (Cols s0 a)’ with ‘Res a’ Expected type: IO [Res a] Actual type: IO [Res (Cols s0 a)] NB: ‘Res’ is a type function, and may not be injective The type variable ‘s0’ is ambiguous • In the expression: withDB $ query (select table) In an equation for ‘list’: list table = withDB $ query (select table) • Relevant bindings include table :: Table a (bound at src/Hedger/Category.hs:44:6) list :: Table a -> IO [Res a] (bound at src/Hedger/Category.hs:44:1) | 44 | list table = withDB $ query (select table) ``` Also, notice that the inferred type does differentiate between `Cols s a` and `a`.
Can you show us a minimal example of `list` working? I'm quite confused about how it could work.
``` type CategoriesSchema = RowID:*:Text type ExpensesSchema = RowID:*:Text:*:Double:*:RowID categories:: Table (CategoriesSchema) (categories, categoryID :*: rest) = tableWithSelectors "categories" $ autoPrimary "id" :*: required "name" expenses:: Table (ExpensesSchema) expenses = table "expenses" $ autoPrimary "id" :*: required "name" :*: required "amount" :*: required "category_id" `fk` (categories, categoryID) withDB (tryCreateTable categories) withDB (tryCreateTable expenses) withDB $ insert_ categories [ def :*: "foo" ] list categories --- [1 :*: "foo"] list expenses --- [] Marc Busqué http://waiting-for-dev.github.io/about/