haskell/esqueleto
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Nicer 'from' interface.

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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
This commit is contained in:
Felipe Lessa 2012-09-03 17:03:07 -03:00
parent 5bdac55e90
commit 2986d0996e
1 changed files with 26 additions and 22 deletions
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48
src/Database/Esqueleto/Internal/Language.hs
View File

@ -56,45 +56,49 @@ infixr 3 &&.
infixr 2 ||.
-- | @FROM@ clause: bring an entity into scope.
--
-- The following types implement 'from':
--
-- * @Expr (Entity val)@, which brings a single entity into scope.
--
-- * Tuples of any other types supported by 'from'. Calling
-- 'from' multiple times is the same as calling 'from' a
-- single time and using a tuple.
--
-- 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 :: From query expr backend a => (a -> query b) -> query b
from = (from_ >>=)
class Esqueleto query expr backend => From query expr backend a where
-- | @FROM@ clause: bring an entity into scope.
--
-- The following types implement 'from':
--
-- * @Expr (Entity val)@, which brings a single entity into scope.
--
-- * Tuples of any other types supported by 'from'. Calling
-- 'from' multiple times is the same as calling 'from' a
-- single time and using a tuple.
--
-- 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_ :: 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_