Nicer 'from' interface.

Instead of

  select $ do
    (x,y,z) <- from
    where_ (z^.f ==. y^.f)
    return (x, y^.f, z)

now you may write

  select $
  from $ \(x,y,z) -> do
    where_ (z^.f ==. y^.f)
    return (x, y^.f, z)

Now the only difference in reading order wrt. SQL is the return
on the bottom of the expression. =)

Note that this does not change at all the expressivity of the
language since

  oldFrom = from return

src/Database/Esqueleto/Internal/Language.hs

@@ -56,7 +56,6 @@ infixr 3 &&.
 infixr 2 ||.
 
 
-class Esqueleto query expr backend => From query expr backend a where
 -- | @FROM@ clause: bring an entity into scope.
 --
 -- The following types implement 'from':
@@ -70,31 +69,36 @@ class Esqueleto query expr backend => From query expr backend a where
 -- Note that using 'from' for the same entity twice does work
 -- and corresponds to a self-join.  You don't even need to use
 -- two different calls to 'from', you may use a tuple.
-  from :: query a
+from :: From query expr backend a => (a -> query b) -> query b
+from = (from_ >>=)
+
+
+class Esqueleto query expr backend => From query expr backend a where
+  from_ :: query a
 
 instance ( Esqueleto query expr backend
          , PersistEntity val
          , PersistEntityBackend val ~ backend
          ) => From query expr backend (expr (Entity val)) where
-  from = fromSingle
+  from_ = fromSingle
 
 instance ( From query expr backend a
          , From query expr backend b
          ) => From query expr backend (a, b) where
-  from = (,) <$> from <*> from
+  from_ = (,) <$> from_ <*> from_
 
 instance ( From query expr backend a
          , From query expr backend b
          , From query expr backend c
          ) => From query expr backend (a, b, c) where
-  from = (,,) <$> from <*> from <*> from
+  from_ = (,,) <$> from_ <*> from_ <*> from_
 
 instance ( From query expr backend a
          , From query expr backend b
          , From query expr backend c
          , From query expr backend d
          ) => From query expr backend (a, b, c, d) where
-  from = (,,,) <$> from <*> from <*> from <*> from
+  from_ = (,,,) <$> from_ <*> from_ <*> from_ <*> from_
 
 instance ( From query expr backend a
          , From query expr backend b
@@ -102,7 +106,7 @@ instance ( From query expr backend a
          , From query expr backend d
          , From query expr backend e
          ) => From query expr backend (a, b, c, d, e) where
-  from = (,,,,) <$> from <*> from <*> from <*> from <*> from
+  from_ = (,,,,) <$> from_ <*> from_ <*> from_ <*> from_ <*> from_
 
 instance ( From query expr backend a
          , From query expr backend b
@@ -111,7 +115,7 @@ instance ( From query expr backend a
          , From query expr backend e
          , From query expr backend f
          ) => From query expr backend (a, b, c, d, e, f) where
-  from = (,,,,,) <$> from <*> from <*> from <*> from <*> from <*> from
+  from_ = (,,,,,) <$> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_
 
 instance ( From query expr backend a
          , From query expr backend b
@@ -121,7 +125,7 @@ instance ( From query expr backend a
          , From query expr backend f
          , From query expr backend g
          ) => From query expr backend (a, b, c, d, e, f, g) where
-  from = (,,,,,,) <$> from <*> from <*> from <*> from <*> from <*> from <*> from
+  from_ = (,,,,,,) <$> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_
 
 instance ( From query expr backend a
          , From query expr backend b
@@ -132,4 +136,4 @@ instance ( From query expr backend a
          , From query expr backend g
          , From query expr backend h
          ) => From query expr backend (a, b, c, d, e, f, g, h) where
-  from = (,,,,,,,) <$> from <*> from <*> from <*> from <*> from <*> from <*> from <*> from
+  from_ = (,,,,,,,) <$> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_ <*> from_