[Haskell-cafe] Selda: confused about type signtures

[Brandon Allbery]

The more precise answer to your question is that an explicit type signature
is taken as exact. If the type needed is some (Ctx a => a), as here, but
your type signature just says a, you will get a type error exactly as you
did.

"a" there does not mean "figure out a type for me". It means *any type at
all*. Including Void, (), Int, etc., which one would not expect to work
there.

On Fri, Apr 20, 2018 at 2:33 PM, Marc Busqué <marc at lamarciana.com> wrote:

> Hi!
>
> I'm using [selda](https://hackage.haskell.org/package/selda) package in
> order to deal with database backends.
>
> Selda uses `TypeOperators` language extension, and it introduces  `:*:`
> type operator constructor which take concrete types like `RowID` or
> `Text` . It also has a `Table` type constructor which takes anything
> built with `:*:`). On the other hand, `SeldaM` is selda custom monad
> transformer with `IO` at the bottom.
>
> I have following helper function which just wraps a call to the SQLite
> backend. `dBPath` is just the database file path:
>
> ```
> withDB :: SeldaM a -> IO a
> withDB act = do
>   path <- dBPath
>   withSQLite path act
> ```
>
> I want to wrap selda API in custom functions to be more resilient to
> changes. Right now I'm trying to abstract selecting all rows for a given
> table (maybe it seems brittle, but it is just a toy project in order to
> learn Haskell):
>
> ```
> list table = withDB $ query (select table) ```
>
> Not adding a type signature to `list` produces following compilation
> error:
>
> ```
> • Non type-variable argument
>     in the constraint: selda-0.1.12.1:Database.Selda.Column.Columns
>                          (selda-0.1.12.1:Database.Selda.Column.Cols s a)
>   (Use FlexibleContexts to permit this)
> • When checking the inferred type
>     list :: forall s a.
>             (selda-0.1.12.1:Database.Selda.Column.Columns
>                (selda-0.1.12.1:Database.Selda.Column.Cols s a),
>              selda-0.1.12.1:Database.Selda.Compile.Result
>                (selda-0.1.12.1:Database.Selda.Column.Cols s a)) =>
>             Table a
>             -> IO
>                  [selda-0.1.12.1:Database.Selda.Compile.Res
>                     (selda-0.1.12.1:Database.Selda.Column.Cols s a)]
> ```
>
> If I try to add what I think would be the correct signature:
>
> ```
> list :: Table a -> IO [a]
> ```
>
> The error changes to:
>
> ```
> • Couldn't match type ‘a’
>                  with ‘selda-0.1.12.1:Database.Selda.Compile.Res
>                          (selda-0.1.12.1:Database.Selda.Column.Cols s0 a)’
>   ‘a’ is a rigid type variable bound by
>     the type signature for:
>       list :: forall a. Table a -> IO [a]
>     at src/Hedger/Category.hs:35:1-25
>   Expected type: SeldaM [a]
>     Actual type: selda-0.1.12.1:Database.Selda.Backend.Internal.SeldaT
>                    IO
>                    [selda-0.1.12.1:Database.Selda.Compile.Res
>                       (selda-0.1.12.1:Database.Selda.Column.Cols s0 a)]
> • In the second argument of ‘($)’, namely ‘query (select table)’
>   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:36:6)
>     list :: Table a -> IO [a] (bound at src/Hedger/Category.hs:36:1)
>    |
> 36 | list table = withDB $ query (select table)
> ```
>
> However, if I constraint the type signature to act just for a given
> table, it compiles without errors:
>
> ```
> type CategoriesSchema = RowID:*:Text
> list :: Table CategoriesSchema -> IO [CategoriesSchema]
> ```
>
> Why is that it works with concrete types but not with a type variable
> that takes the same concrete type in its both placeholders?
>
> Thanks in advance,
>
> Marc Busqué
> http://waiting-for-dev.github.io/about/
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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