6.2.5. Qualified do-notation
- QualifiedDo
- Since:
9.0.1
Allow the use of qualified
donotation.
QualifiedDo enables qualifying a do block with a module name, to control which operations to use for
the monadic combinators that the do notation desugars to.
When -XQualifiedDo is enabled, you can qualify the do notation by writing modid.do, where
modid is a module name in scope:
{-# LANGUAGE QualifiedDo #-}
import qualified Some.Module.Monad as M
action :: M.SomeType a
action = M.do x <- u
res
M.return x
The additional module name (here M) is called the qualifier of the do-expression.
The unqualified do syntax is convenient for writing monadic code, but
it only works for data types that provide an instance of the Monad type class.
There are other types which are “monad-like” but can’t provide an instance of
Monad (e.g. indexed monads, graded monads or relative monads), yet they could
still use the do syntax if it weren’t hardwired to the methods of the Monad
type class. -XQualifiedDo comes to make the do syntax customizable in this
respect.
It allows you to mix and match do blocks of different types with suitable
operations to use on each case:
{-# LANGUAGE QualifiedDo #-}
import qualified Control.Monad.Linear as L
import MAC (label, box, runMAC)
import qualified MAC as MAC
f :: IO ()
f = do
x <- runMAC $ -- (Prelude.>>=)
-- (runMAC $
MAC.do --
d <- label "y" -- label "y" MAC.>>= \d ->
box $ --
-- (box $
L.do --
r <- L.f d -- L.f d L.>>= \r ->
L.g r -- L.g r L.>>
L.return r -- L.return r
-- ) MAC.>>
MAC.return d -- (MAC.return d)
-- )
print x -- (\x -> print x)
The semantics of do notation statements with -XQualifiedDo is as follows:
The
x <- ustatement uses(M.>>=)M.do { x <- u; stmts } = u M.>>= \x -> M.do { stmts }
The
ustatement uses(M.>>)M.do { u; stmts } = u M.>> M.do { stmts }
The a
pat <- ustatement usesM.failfor the failing case, if such a case is neededM.do { pat <- u; stmts } = u M.>>= \case { pat -> M.do { stmts } ; _ -> M.fail "…" }
If the pattern cannot fail, then we don’t need to use
M.fail.M.do { pat <- u; stmts } = u M.>>= \case pat -> M.do { stmts }
The desugaring of
-XApplicativeDousesM.fmap,(M.<*>), andM.join(after the the applicative-do grouping has been performed)M.do { (x1 <- u1 | … | xn <- un); M.return e } = (\x1 … xn -> e) `M.fmap` u1 M.<*> … M.<*> un M.do { (x1 <- u1 | … | xn <- un); stmts } = M.join ((\x1 … xn -> M.do { stmts }) `M.fmap` u1 M.<*> … M.<*> un)
Note that
M.joinis only needed if the final expression is not identifiably areturn. With-XQualifiedDoenabled,-XApplicativeDolooks only for the qualifiedreturn/purein a qualified do-block.
With
-XRecursiveDo,recandmdoblocks useM.mfixandM.return:M.do { rec { x1 <- u1; … ; xn <- un }; stmts } = M.do { (x1, …, xn) <- M.mfix (\~(x1, …, xn) -> M.do { x1 <- u1; …; xn <- un; M.return (x1, …, xn)}) ; stmts }
If a name M.op is required by the desugaring process (and only if it’s required!) but the name is
not in scope, it is reported as an error.
The types of the operations picked for desugaring must produce an expression which is accepted by the typechecker. But other than that, there are no specific requirements on the types.
If no qualifier is specified with -XQualifiedDo enabled, it defaults to the operations defined in the Prelude, or, if
-XRebindableSyntax is enabled, to whatever operations are in scope.
Note that the operations to be qualified must be in scope for QualifiedDo to work. I.e. import MAC (label) in the
example above would result in an error, since MAC.>>= and MAC.>> would not be in scope.
6.2.5.1. Examples
-XQualifiedDo does not affect return in the monadic do notation.
import qualified Some.Monad.M as M
boolM :: (a -> M.M Bool) -> b -> b -> a -> M.M b
boolM p a b x = M.do
px <- p x -- M.>>=
if px then
return b -- Prelude.return
else
M.return a -- M.return
-XQualifiedDo does not affect explicit (>>=) in the monadic do notation.
import qualified Some.Monad.M as M
import Data.Bool (bool)
boolMM :: (a -> M.M Bool) -> M b -> M b -> a -> M.M b
boolMM p ma mb x = M.do
p x >>= bool ma mb -- Prelude.>>=
Nested do blocks do not affect each other’s meanings.
import qualified Some.Monad.M as M
f :: M.M SomeType
f = M.do
x <- f1 -- M.>>=
f2 (do y <- g1 -- Prelude.>>=
g2 x y)
where
f1 = ...
f2 m = ...
g1 = ...
g2 x y = ...
The type of (>>=) can also be modified, as seen here for a graded monad:
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Control.Monad.Graded (GradedMonad(..)) where
import Data.Kind (Constraint)
class GradedMonad (m :: k -> * -> *) where
type Unit m :: k
type Plus m (i :: k) (j :: k) :: k
type Inv m (i :: k) (j :: k) :: Constraint
(>>=) :: Inv m i j => m i a -> (a -> m j b) -> m (Plus m i j) b
return :: a -> m (Unit m) a
-----------------
module M where
import Control.Monad.Graded as Graded
g :: GradedMonad m => a -> m SomeTypeIndex b
g a = Graded.do
b <- someGradedFunction a Graded.>>= someOtherGradedFunction
c <- anotherGradedFunction b
Graded.return c