[Haskelldb-users] Converting BoundedString to String

[Justin B. <jgb...@gm...>]

I have run into a problem several times where I have a column that is
defined as Expr BStr10 (or similar) and I want to convert it to a
string column, i.e. Expr String. I came up with a class declaration
and some instances that can do it, but is there a simpler way? I have
(which must be defined in Query.hs since it uses the Expr
constructor):

class BStrToStr s d where
  toStr :: s -> d

instance (Size n) => BStrToStr (Expr (BoundedString n)) (Expr String) where
  toStr (Expr e) = Expr e

instance (Size n) => BStrToStr (Expr (Maybe (BoundedString n))) (Expr
(Maybe String)) where
  toStr (Expr m) = Expr m

This toy example shows the class in action. "query" will compile fine,
but "queryX" will not (when uncommented). Any suggestions are welcome!

Justin

\begin{code}
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
import Database.HaskellDB
import Database.HaskellDB.DBLayout
import Database.HaskellDB.Query (Expr(..))
import Database.HaskellDB.BoundedList (Size)

query = do
 t <- table someTable
 project $ strField << (toStr (t ! bstrField) :: Expr String)

-- Below does not compile.
--
queryX = do
  t <- table someTable
  project $ strField << t ! bstrField

class BStrToStr s d where
  toStr :: s -> d

instance (Size n) => BStrToStr (Expr (BoundedString n)) (Expr String) where
  toStr (Expr e) = Expr e

instance (Size n) => BStrToStr (Expr (Maybe (BoundedString n))) (Expr
(Maybe String)) where
  toStr (Expr m) = Expr m

type SomeTable = (RecCons StrField (Expr String)
                   (RecCons BStrField (Expr BStr10) RecNil))

someTable :: Table SomeTable
someTable = baseTable "someTable" $
             hdbMakeEntry StrField #
             hdbMakeEntry BStrField

data StrField = StrField
instance FieldTag StrField where
    fieldName _ = "strField"

strField :: Attr StrField String
strField = mkAttr StrField

data BStrField = BStrField
instance FieldTag BStrField where
    fieldName _ = "bstrField"

bstrField :: Attr BStrField BStr10
bstrField = mkAttr BStrField

\end{code}