John Hughes & Magnus Carlsson
A more detailed treatment of the art of combining monads is found in [LHJ95] (PostScript) , which also uses Gofer to give a more elegant presentation by the use of constructor classes.
type I a = a unit v = v x `bind` f = f xThe identity monad represents computations as the values they deliver. It can support no interesting operations, but is the starting point to which other capabilities can be added.
Assume that we already have a monad M
, with operations unitM
and
bindM
. We can then define new monads extending the capabilities
of M
. This is done by defining a new bind
operation, in
terms of bindM
, and a function liftNew
,
liftNew :: M a > MNew awhich turns operations in the underlying monad into the new monad. Not all monad operations can be lifted in this simple way, but most of them. In particular, the new
unit
can be expressed as
unitNew a :: MNew a unitNew a = liftNew (unitM a)
type MSt s a = s > M (a,s) x `bindSt` f = \s > x s `bindM` \(v,s') > f v s' liftSt :: M a > MSt s a liftSt m = \s > m `bindM` \v > unitM (v,s)Useful operations include
fetchSt :: MSt s s fetchSt = \s > unitM (s,s) storeSt :: s > MSt s () storeSt s = \_ > unitM ((),s)One useful application of state monads is to manage input. Let the state be the list of input tokens remaining to be read. Then we can read a token with
get :: MSt [t] t get = fetchSt `bindSt` \(t:ts) > storeSt ts `bindSt` \_ > liftSt (unitM t)
The `state reader' monad manages a state that is never updated  perhaps
better thought of as a hidden parameter. You might use it if you want access
to, say, the list of command line arguments, without passing it explicitly as
a parameter throughout your program. It can also be used for
recording variable environments which may be extended for
subcomputations by means of extendSR
.
type MSR s a = s > M a x `bindSR` f = \s > x s `bindM` \v > f v s liftSR :: M a > MSR s a liftSR m = \s > m fetchSR :: MSR s s fetchSR = unitM extendSR :: s > MSR s a > MSR s a extendSR s m = \_ > m s
Given any associative operator
(<+>) :: t > t > twith unit element
z
, we can define
a monad by
type MMon t a = M (a,t) x `bindMon` f = x `bindM` \~(v,a) > f v `bindM` \~(w,b) > unitM (w,a<+>b) liftMon :: M a > MMon t a liftMon m = m `bindM` \v > unitM (v,z) putMon :: t > MMon () putMon t = unitM ((),t)The
~
symbols on the pair patterns make pattern matching lazy.
An important application is when the type t
is a list of outputs
produced. Here the operator <+>
is list concatenation (++
),
and its unit is the empty list. This application motivates the lazy
pattern matching in bindMon
: it's important that the output is produced
lazily, in particular that the output from the first subcomputation can be
produced before the second subcomputation is started. Thanks to the
second ~
, the result of f v
isn't required until either
we try to evaluate the result w
, or the output b
from f v
.
type MEx e a = M (Error e a) data Error e a = Yes a  No e x `bindEx` f = x `bindM` \y > case y of Yes v > f v No e > unitM (No e) liftEx m = m `bindM` \v > unitM (Yes v) unitEx v = unitM (Yes v)Operations to raise and handle exceptions can be defined as shown:
raiseEx :: e > MEx e a raiseEx e = unitM (No e) handleEx :: MEx e' a > (e' > MEx e a) > MEx e a x `handleEx` h = x `bindM` \y > case y of Yes v > unitM (Yes v) No e > h eOf course the exception value
e
can be omitted if not needed.
It can make a big difference if the exception monad is added on top of
M
, or if M
is added on top of the exception monad. Try
to build a state monad on top of an exception monad, and vice
versa. What is the difference?
Backtracking can be added to any monad in a similar way:
type MBt a = M [a] x `bindBt` f = x `bindM` \vs > foldr orelseBt failBt (map f vs) liftBt :: M a > MBt a liftBt m = m `bindM` \v > unitM [v] infixl 5 `apM` apM :: M (a > b) > M a > M b fm `apM` fx = fm `bindM` \f > fx `bindM` \x > unitM (f x) orelseBt :: MBt a > MBt a > MBt a x `orelseBt` y = unitM (++) `apM` x `apM` y failBt :: MBt a failBt = unitM []But this monad lets us only perform one
M
computation to
produce a list of results. We might instead want to perform an
M
computation for each new possible answer. To do this we
need a recursive monad:
type MRBt a = M (Ans a) data Ans a = Ans a (MRBt a)  NoAns x `bindRBt` f = x `bindM` \y > case y of NoAns > failRBt Ans v x' > f v `orelseRBt` (x' `bindRBt` f) liftRBt m = m `bindM` \v > unitM (Ans v failRBt) failRBt = unitM NoAns x `orelseRBt` y = x `bindM` \z > case z of NoAns > y Ans v x' > unitM (Ans v (x' `orelseRBt` y))
If we want to build monads on top of a continuation based programming paradigm, such as stream processors or continuation based I/O in Haskell, we need to build a monad around the continuation based operations. The monad type reflects that every computation takes a continuation as a parameter, and applies it to the value to get a final result:
type MCio r a = (a > r) > r unitCio v = \k > k v x `bindCio` f = \k > x (\v > f v k)Typically, in continuation based programming, one doesn't have a
bind

operation, but instead two different kinds of operations. One is used
for doing sideeffects, and these have the type
r > rThe other kind of operation passes information (of type
i
) to
the continuation, and the type of these is
(i > r) > rWe need to lift these two kinds of operations into monadic operations:
liftc :: (r > r) > MCio r () liftc d = \k > d (k ()) lifti :: ((a > r) > r) > MCio r a lifti d = d
Here are examples of the two kinds of operations, from stream processors and dialogue I/O:
putCio :: o > MCio (SP i o) () putCio o = liftc (putSP o) getCio :: MCio (SP i a) i getCio = lifti getSP writeFileCio :: String > String > MCio Dialogue () writeFileCio file content = liftc (writeFile file content exit) readFileCio :: String > MCio Dialogue String readFileCio f = lifti (readFile f exit)
A continuation monad also let us model control operations similar to goto in imperative languages. To do this, we need an operation to catch the continuation and return it as a value, and an operation to jump to a continuation (see the example Devils and Angels for an application of these operations):
callccCio :: ((a > r) > MCio r a) > MCio r a callccCio f = \k > f k k jumpCio :: (a > r) > a > MCio r b jumpCio k v = \_ > k v
The operations callcc
and jump
could be useful to add to
any monad. The continuation monad can in fact be added to any monad,
by letting the result type be a monad value. In addition to the above
operations, we only need the lift operation:
liftCio :: M a > MCio (M r) a liftCio m = \k > m `bindM` \v > k v
Suppose we want to do dialogue I/O in our monad, as well as manipulate state and handle exceptions on top of that. Combining the types given above, we get the type
type MSt d s a = s > MCio d (a,s) type MEx e d s a = MSt d s (Error e a) type MyMonad a = MEx MyError Dialogue MyState a type MyError = ... type MyState = ...If we want, we can rename the operations in the topmost monad if we don't want to reveal the structure of it:
bind :: MyMonad a > (a > MyMonad b) > MyMonad b bind = bindEx raise :: MyError > MyMonad a raise = raiseEx handle :: MyMonad a > (MyError > MyMonad a) > MyMonad a handle = handleExOperations from the lower monads need to be lifted:
unit :: a > MyMonad a unit = liftEx . liftSt . unitCio fetch :: MyMonad MyState fetch = liftEx fetchSt store :: MyState > MyMonad MyState store s = liftEx (storeSt s) dodia :: (Dialogue > Dialogue) > MyMonad () dodia = liftEx . liftSt . liftc getdia :: ((a > Dialogue) > Dialogue) > MyMonad a getdia = liftEx . liftSt . liftiHow can we invoke a computation of type
MyMonad a
? By expanding
the type, we get
type MyMonad a = MyState > ((Error MyError a,MyState) > Dialogue) > Dialoguewhich indicates a suitable way of invoking it on the toplevel in a program:
invoke :: MyMonad a > MyState > Dialogue invoke m startstate = m startstate (\_ > done)Here, we ignore the final state, as well as the final (error)value.
Systematic Design of Monads
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