Better documentation of Sql's internals.

src/Database/Esqueleto/Internal/Sql.hs

@@ -21,6 +21,8 @@ module Database.Esqueleto.Internal.Sql
   , update
   , toRawSql
   , Mode(..)
+  , Escape
+  , SqlSelect
   ) where
 
 import Control.Applicative (Applicative(..), (<$>))
@@ -199,6 +201,7 @@ parensM Parens = parens
 
 data OrderByType = ASC | DESC
 
+-- | (Internal) Backend-specific function that escapes a 'DBName'.
 type Escape = DBName -> TLB.Builder
 
 
@@ -451,7 +454,12 @@ update = rawExecute UPDATE . from
 ----------------------------------------------------------------------
 
 
--- | Pretty prints a 'SqlQuery' into a SQL query.
+-- | (Internal) Pretty prints a 'SqlQuery' into a SQL query.
+--
+-- Note: if you're curious about the SQL query being generated by
+-- @esqueleto@, instead of manually using this function (which is
+-- possible but tedious), you may just turn on query logging of
+-- @persistent@.
 toRawSql :: SqlSelect a r => Mode -> Escape -> SqlQuery a -> (TLB.Builder, [PersistValue])
 toRawSql mode esc query =
   let (ret, SideData fromClauses setClauses whereClauses orderByClauses) =
@@ -466,6 +474,7 @@ toRawSql mode esc query =
       , makeOrderBy esc orderByClauses
       ]
 
+-- | (Internal) Mode of query being converted by 'toRawSql'.
 data Mode = SELECT | SELECT_DISTINCT | DELETE | UPDATE
 
 
@@ -552,8 +561,11 @@ parens :: TLB.Builder -> TLB.Builder
 parens b = "(" <> (b <> ")")
 
 
--- | Class for mapping results coming from 'SqlQuery' into actual
--- results.
+----------------------------------------------------------------------
+
+
+-- | (Internal) Class for mapping results coming from 'SqlQuery'
+-- into actual results.
 --
 -- This looks very similar to @RawSql@, and it is!  However,
 -- there are some crucial differences and ultimately they're
@@ -571,11 +583,15 @@ class SqlSelect a r | a -> r, r -> a where
   -- | Transform a row of the result into the data type.
   sqlSelectProcessRow :: [PersistValue] -> Either T.Text r
 
+
+-- | Not useful for 'select', but used for 'update' and 'delete'.
 instance SqlSelect () () where
   sqlSelectCols _ _ = mempty
   sqlSelectColCount _ = 0
   sqlSelectProcessRow _ = Right ()
 
+
+-- | You may return an 'Entity' from a 'select' query.
 instance PersistEntity a => SqlSelect (SqlExpr (Entity a)) (Entity a) where
   sqlSelectCols escape expr@(EEntity ident) = ret
       where
@@ -602,6 +618,8 @@ instance PersistEntity a => SqlSelect (SqlExpr (Entity a)) (Entity a) where
 getEntityVal :: SqlExpr (Entity a) -> a
 getEntityVal = error "Esqueleto/Sql/getEntityVal"
 
+
+-- | You may return a possibly-@NULL@ 'Entity' from a 'select' query.
 instance PersistEntity a => SqlSelect (SqlExpr (Maybe (Entity a))) (Maybe (Entity a)) where
   sqlSelectCols escape (EMaybe ent) = sqlSelectCols escape ent
   sqlSelectColCount = sqlSelectColCount . fromEMaybe
@@ -612,6 +630,9 @@ instance PersistEntity a => SqlSelect (SqlExpr (Maybe (Entity a))) (Maybe (Entit
     | all (== PersistNull) cols = return Nothing
     | otherwise                 = Just <$> sqlSelectProcessRow cols
 
+
+-- | You may return any single value (i.e. a single column) from
+-- a 'select' query.
 instance PersistField a => SqlSelect (SqlExpr (Value a)) (Value a) where
   sqlSelectCols esc (ERaw p f) = let (b, vals) = f esc
                                  in (parensM p b, vals)
@@ -619,6 +640,9 @@ instance PersistField a => SqlSelect (SqlExpr (Value a)) (Value a) where
   sqlSelectProcessRow [pv] = Value <$> fromPersistValue pv
   sqlSelectProcessRow _    = Left "SqlSelect (Value a): wrong number of columns."
 
+
+-- | You may return tuples (up to 8-tuples) and tuples of tuples
+-- from a 'select' query.
 instance ( SqlSelect a ra
          , SqlSelect b rb
          ) => SqlSelect (a, b) (ra, rb) where
@@ -643,6 +667,7 @@ instance ( SqlSelect a ra
     in colCountFst `seq` processRow
        -- Avoids recalculating 'colCountFst'.
 
+
 instance ( SqlSelect a ra
          , SqlSelect b rb
          , SqlSelect c rc