Re: [Haskell-cafe] state and exception or types again...
Il Fri, Sep 01, 2006 at 10:43:14AM +0400, Bulat Ziganshin ebbe a scrivere: imho, your tutorial makes the error that is a very typical: when you write your tutorial you already know what are monads and what the program you will construct at the end. but your reader don't know all these! Ok, so I did it again. Try with: Meet Bob The Monadic Lover http://www.haskell.org/haskellwiki/Meet_Bob_The_Monadic_Lover the last part, the real monadic, is to be written, but I had very limited time. I will, though. let me know, please. andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] state and exception or types again...
Hello Andrea, Thursday, August 31, 2006, 4:22:49 PM, you wrote: The tutorial will have this outline: first we build a monad adding output, exception, and state. Then we use monad transformer to take out state and output and add debug, doing lifting, put(ing) and get(ing) by hand, to understand the central role of type matching/construction. imho, your tutorial makes the error that is a very typical: when you write your tutorial you already know what are monads and what the program you will construct at the end. but your reader don't know all these! for such fresh reader this looks as you made some strange steps, write some ugly code and he don't have chances to understand that this ugly code is written just to show that this can be simplified using monads. i've tried to read it imaging myself as fresh reader and was stopped at some middle because code was too complicated to understand and it was completely imobvious (for fresh reader) that we just wrote innards of monad and then will reduce all this ugly code just to = calls -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] state and exception or types again...
Il Fri, Sep 01, 2006 at 10:43:14AM +0400, Bulat Ziganshin ebbe a scrivere: The tutorial will have this outline: first we build a monad adding output, exception, and state. Then we use monad transformer to take out state and output and add debug, doing lifting, put(ing) and get(ing) by hand, to understand the central role of type matching/construction. imho, your tutorial makes the error that is a very typical: when you write your tutorial you already know what are monads and what the program you will construct at the end. but your reader don't know all these! Neither did I, actually. for such fresh reader this looks as you made some strange steps, write some ugly code and he don't have chances to understand that this ugly code is written just to show that this can be simplified using monads. i've tried to read it imaging myself as fresh reader and was stopped at some middle because code was too complicated to understand and it was completely imobvious (for fresh reader) that we just wrote innards of monad and then will reduce all this ugly code just to = calls I do not entirely understand your point. I wrote just the first part of the tutorial, till the Errare Monadicum Est chapter. From then on, before writing the tutorial, I needed to understand what I was headed to and so I wrote the code. Now the task is to explain each step of that code. Indeed I'm a fresh reader that did not find anything that she could find useful to understand monads, and wrote her own. That is to say, this is the way I came to understand monads. I do not pretend to teach what monads are, but I hope that, by following the same path I followed, someone else can get to the point where I am now. Where now I am is just another kind of problem. Probably nowhere. That's it. Anyway, thanks a lot for your suggestions. I'll try to understand them and put them in practice. As far as I can. Best regards, Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] state and exception or types again...
Il Wed, Aug 30, 2006 at 11:24:45AM +0100, Brian Hulley ebbe a scrivere:
Thanks, glad to be of help.
I met Haskell a couple of months ago, when I switched my window
manager to Ion. Tuomo Valkonen, its developer, uses darcs. Moreover he
develops a small PIM, riot, written in Haskell. I wanted to play
around with it but the source code was literally unreadable, for me. I
could not understand a single line.
I do some php. Lately I discovered Javascript and Lua, and I fell in
love with their object-oriented capabilities. I'm not a real coder
(though I develop a wiki in php), but I like getting to know new
programming practices. I've been intrigue with functional programming
for quite sometime but I just knew a bit of Scheme, because it's used
as a scripting language in LilyPond (a package for writing music
sheets).
So I decided to spend my August holidays to study Haskell. I started
reading some tutorial but could not understand what was going on. I
then thought to take the long way: I watched the Abelson and Sussman's
lectures and used ghci to (sort of) follow what they were doing.
Coming from dynamic typed languages (I know very little C) the type
system was horrible. Monads seemed mysterious objects.
I've read in the Yet Another... that the Haskell community is very
supportive.
With your help I have now a different perspective. Playing with
Haskell is like completing a puzzle, whose pieces' shapes are made up
with rules that, at first, you seem not be able to grasp.
You keep pushing the last piece, and it just doesn't fit in.
Then you realize that shapes are actually types. And when you start
understanding the rules of type construction and type matching, now
you can recognize the shapes of the pieces you are playing with. And
start making rational guesses on where they should go.
Now, when I see Compiling ... Ok, modules loaded: I feel like I
ended up solving the puzzle.
It's an amazing pleasure... Haskell and functional programming.
Without you I couldn't do it. Thank you so much!
The tutorial will have this outline: first we build a monad adding
output, exception, and state. Then we use monad transformer to take
out state and output and add debug, doing lifting, put(ing) and
get(ing) by hand, to understand the central role of type
matching/construction.
We end up with the following code, that should clearly show all the
previous (hand made) steps should lead. No lambda calculus inhere!
Once again, thanks!
Andrea
-The Final Evaluator
module MyStateT where
import Control.Monad.State hiding (State)
data Term = Con Int
| Add Term Term
deriving (Show)
type IOStack = [Output]
type Output = String
type Debug = [String]
data EvalST = State {getIOS :: IOStack, getDebug :: Debug, getCount:: Int}
deriving(Show)
type Exception = String
data MT a = Fail Exception
| Done {unpackDone :: a }
deriving (Show)
type Eval s a = StateT s MT a
instance Monad MT where
return a = Done a
m = f = case m of
Fail e - Fail e
Done a - f a
instance Functor MT where
fmap _ (Fail a) = Fail a
fmap f (Done a) = Done (f a)
emptyState = State [] [] 0
stopExecT exc = lift $ Fail exc
catchT e = do st - get
let s = getCount st
let es = getDebug st
let o = getIOS st
let exc = Debug msg at Iteration ++ show s ++ : ++ e
put $ State o (exc:es) s
printT :: Output - Eval EvalST ()
printT o = do st - get
let s = getCount st
let e = getDebug st
let os = getIOS st
let out = show s ++ - ++ o
put $ State (out:os) e s
incTcounter :: Eval EvalST ()
incTcounter = do st - get
let s = getCount st
let e = getDebug st
let o = getIOS st
put $ State o e (s+1)
evalT :: Term - Eval EvalST Int
evalT (Con a) = do incTcounter
printT (formatLine (Con a) a)
return a
evalT (Add t u) = do a - evalT t
b - evalT u
incTcounter
let out = formatLine (Add t u) (a + b)
printT out
case (a+b) of
42 - do catchT The Ultimate Answer Has Been Computed!!
Now I'm tired!
return (a+b)
11 - stopExecT 11 I do not like this number!
otherwise - return (a + b)
formatLine :: Term - Int - Output
formatLine t a = eval ( ++ show t ++ ) = ++ show a
printAll :: [String] - IO ()
printAll [] = return ()
printAll (a:xs) = do print a
printAll xs
eval :: Term - IO ()
eval exp = case execStateT (evalT exp) emptyState of
Fail e - print e
Done (State a b c )
- do printAll $ reverse a
Re: [Haskell-cafe] state and exception or types again...
Il Tue, Aug 29, 2006 at 10:02:38AM +0100, Brian Hulley ebbe a scrivere: Hi! Ím getting back so late because it took me a while to understand monadic transformation... data Result a = Good a State Output | Bad State Output deriving Show ... case runSOIE m x of Good a y o1 - case runSOIE (f a) y of Good b z o2 - Good b z (o1 ++ o2) Bad z o2 - Bad z (o1 ++ o2) Bad z o2 - Bad z o2 -- (*) This is brilliant and highly instructive (for me at least)!! Thank you very much. Regards, Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] state and exception or types again...
Andrea Rossato wrote:
Il Mon, Aug 28, 2006 at 09:28:02PM +0100, Brian Hulley ebbe a
scrivere:
where the 4th element of the tuple is True iff we can continue or
False iff an exception occurred.
I'm starting to believe that the best method is just take the way
StateT takes... without reinventing the wheel...
The solution I gave was very close to being correct. I enclose a tested
example below - you'll need to adapt it to do evaluation but it shows an
exception being raised.
module Test where
import Control.Monad
-- When we raise an exception we use (undefined) so that
-- the result type is the same as whatever the result type
-- would be for the other computation. But this means we
-- need to tell Haskell how to print out the tuple so that it
-- doesn't give an exception when trying to print out
-- undefined (!), hence we replace the tuple with a data type
-- so we can define our own Show instance
data Result a = Result a State Output Bool
instance Show a = Show (Result a) where
show (Result a s o True) =
Good ++ show a ++ ++ show s ++ ++ show o
show (Result _ s o _) =
Bad ++ show s ++ ++ show o
-- We only have one constructor so can use a newtype for
-- efficiency
newtype Eval_SOI a = SOIE {runSOIE :: State - Result a}
type State = Int
type Output = String
-- I used braces instead of parens in my previous post
-- Note that we return undefined as the result because this
-- is the only value which belongs to all types in Haskell
raise e = SOIE (\s - Result undefined s e False)
instance Monad Eval_SOI where
return a = SOIE (\s - Result a s True)
m = f = SOIE $ \x -
let
Result a y o1 ok1 = runSOIE m x
in if ok1
then
let
Result b z o2 ok2 = runSOIE (f a) y
in Result b z (o1 ++ o2) ok2
else Result undefined y o1 False
display t = SOIE(\s - Result () s t True)
test = runSOIE (do
display hello
raise Exception
display Foo
) 0
In the definition of (=), we need to explicitly return (undefined) when
the first computation has raised an exception, so that the result type
unifies with the result type when no exception occurs.
Regards, Brian.
--
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.
http://www.metamilk.com
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Re: [Haskell-cafe] state and exception or types again...
Il Tue, Aug 29, 2006 at 07:45:46AM +0100, Brian Hulley ebbe a scrivere: Andrea Rossato wrote: Il Mon, Aug 28, 2006 at 09:28:02PM +0100, Brian Hulley ebbe a scrivere: where the 4th element of the tuple is True iff we can continue or False iff an exception occurred. I'm starting to believe that the best method is just take the way StateT takes... without reinventing the wheel... The solution I gave was very close to being correct. I enclose a tested example below - you'll need to adapt it to do evaluation but it shows an exception being raised. I said I think that the StateT approach is the one to take only because I believe that the complexity of the definition of = is getting unmanageable, that is, as far as I understand, contrary to the spirit of haskell, and functional programming in general. so, start getting my hands dirty in monadic combinations is probably the best for improving my knowledge of haskell and functional programming. what do you think? Thank you very much for your kind attention and the greatly illuminating examples! Best regards, Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] state and exception or types again...
Andrea Rossato wrote:
Il Mon, Aug 28, 2006 at 09:28:02PM +0100, Brian Hulley ebbe a scrivere:
data Eval_SOI a = SOIE {runSOIE :: State - (a, State, Output, Bool)}
well, I thought that this was not possible:
(=) :: m a - (a - m b) - m b
And you are right. In case of an exception, you don't have a 'b' to
return, so you cannot construct the result (unless you put 'undefined'
in there, which is just silly). Do it this way:
data Eval_SOI a = SOIE {runSOIE :: State - (Maybe a, State, Output)}
instance Monad Eval_SOI where
return a = SOIE $ \s - (Just a, s, [])
fail _ = SOIE $ \s - (Nothing, s, [])
m = k = SOIE $ \s0 -
let r@(ma, s1, o1) = runSOIE m s0
(mb, s2, o2) = runSOIE (k (fromJust ma)) s1
in case ma of Nothing - r
Just _ - (mb, s2, o1 ++ o2)
output w = SOIE $ \s - (Just (), s, w)
put s = SOIE $ \_ - (Just (), s, [])
get = SOIE $ \s - (Just s, s, [])
I don't think it's unmanageably complicated, but still not as clean and
modular as using monad transformers.
This is why I think that two constructors are needed, but with two
constructors is not possible...;-)
Indeed. Here they are Nothing and Just. In principle, Maybe is
equivalent to a pair of a Bool and something else, but that only works
in an untyped language.
I'm trying to dig into this problem also to see if it has to do with
monad laws.
Uhh... no. You should prove them, though. (Try it, doing this is quite
instructive.)
Udo.
--
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clear that science can help them; that science should try is not clear
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Re: [Haskell-cafe] state and exception or types again...
Il Tue, Aug 29, 2006 at 10:02:38AM +0100, Brian Hulley ebbe a scrivere:
Yes I agree the StateT/monad transformer approach is probably best in the
long run, since by using the standard monad transformers, you will get code
that will scale better to handle more complexities later, and has the
advantage of being already tested so you can be sure the resulting monads
will obey all the monad laws. Also, there are a lot of tutorials about how
to use them to solve different problems.
...
Hi!
It's been quite troublesome since there are no examples (at least I
did not find any), but I implemented (copied...;-) StateT.
Below the code. What do you think (apart from names or lack of class
instance: I need this code to understand what's going on and to write
about it in my tutorial)?
Is it quite canonical? Anything really bad?
Happy monadic explorations! :-)
Great fun indeed!
Thanks for the kind help from you, guys!
Andrea
Here's the bit. At the end the output.
module Monadi where
data Term = Con Int
| Add Term Term
deriving (Show)
eval :: Term - Int
eval (Con a) = a
eval (Add a b) = eval a + eval b
answer, noanswer :: Term
answer = (Add (Add (Con 30) (Con 12)) (Add (Con 20) (Con 30)))
noanswer = (Add (Add (Con 20) (Con 12)) (Con 11))
formatLine t a = eval ( ++ show t ++ ) = ++ show a ++ -
type Exception = String
type O = String
data M2 a = Ex Exception
| Done {unpack :: (a,O) }
deriving (Show)
newtype StateT s m a = S {runStateT :: s - m (a,s) } --S (s - m (a,s))
instance Monad m = Monad (StateT s m) where
return a = S (\s - return (a,s))
S m1 = k= S (\s - do ~(a,s1) - m1 s
let S m2 = k a
m2 s1)
instance Monad M2 where
return a = Done (a, )
m = f = case m of
Ex e - Ex e
Done (a, x) - case (f a) of
Ex e1 - Ex e1
Done (b, y) - Done (b, x ++ y)
lift m = S (\s - do x - m
return (x,s))
raise_IOE :: O - StateT s M2 a
raise_IOE e = lift (Ex e)
print_IOE :: O - StateT Int M2 ()
print_IOE x = lift (Done ((), x))
incState :: StateT Int M2 (M2 ())
incState = S (\s - return (Done ((), ), s + 1))
eval_IOE :: Term - StateT Int M2 Int
eval_IOE (Con a) = do incState
print_IOE (formatLine (Con a) a)
return a
eval_IOE (Add t u) = do a - eval_IOE t
b - eval_IOE u
incState
print_IOE (formatLine (Add t u) (a + b))
if (a+b) == 42
then raise_IOE The Ultimate Answer Has Been
Computed!! Now I'm tired!
else return (a + b)
-- *Monadi runStateT (eval_IOE answer) 0
-- Ex The Ultimate Answer Has Been Computed!! Now I'm tired!
-- *Monadi runStateT (eval_IOE noanswer) 0
-- Done {unpack = ((43,5),eval (Con 20) = 20 - eval (Con 12) = 12 - eval
(Add (Con 20) (Con 12)) = 32 - eval (Con 11) = 11 - eval (Add (Add (Con 20)
(Con 12)) (Con 11)) = 43 - )}
-- *Monadi
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[Haskell-cafe] state and exception or types again...
Hello!
Sorry if I keep bothering, but I'm still trying to understand types and
monads.
Now I'm trying to create a statefull evaluator, with output and
exception, but I'm facing a problem I seem not to be able to
conceptually solve.
Take the code below.
Now, in order to get it run (and try to debug) the Eval_SOI type has a
Raise constructor that produces the same type of SOIE. Suppose instead it
should be constructing something like Raise something.
Moreover, I wrote a second version of =, commented out.
This is just to help me illustrate to problem I'm facing.
Now, = is suppose to return Raise if m is matched against Raise
(second version commented out).
If m matches SOIE it must return a SOIE only if f a does not
returns a Raise (output must be concatenated).
I seem not to be able to find a way out. Moreover, I cannot understand
if a way out can be possibly found. Something suggests me it could be
related to that Raise something.
But my feeling is that functional programming could be something out
of the reach of my mind... by the way, I teach Law, so perhaps you'll
forgive me...;-)
If you can help me to understand this problem all I can promise is
that I'll mention your help in the tutorial I'm trying to write on
the monadic way... that seems to lead me nowhere.
Thanks for your kind attention.
Andrea
the code:
data Eval_SOI a = Raise { unPackMSOIandRun :: State - (a, State, Output) }
| SOIE { unPackMSOIandRun :: State - (a, State, Output) }
instance Monad Eval_SOI where
return a = SOIE (\s - (a, s, ))
m = f = SOIE (\x -
let (a, y, s1) = unPackMSOIandRun m x in
case f a of
SOIE nextRun - let (b, z, s2) = nextRun y in
(b, z, s1 ++ s2)
Raise e1 - e1 y --Is it right?
)
-- (=) m f = case m of
--Raise e - error ciao -- why this is not going to
happen?
--SOIE a - SOIE (\x -
--let (a, y, s1) = unPackMSOIandRun m x in
--let (b, z, s2) = unPackMSOIandRun (f a) y
in
--(b, z, s1 ++ s2))
incSOIstate :: Eval_SOI ()
incSOIstate = SOIE (\s - ((), s + 1, ))
print_SOI :: Output - Eval_SOI ()
print_SOI x = SOIE (\s - ((),s, x))
raise x e = Raise (\s - (x,s,e))
eval_SOI :: Term - Eval_SOI Int
eval_SOI (Con a) = do incSOIstate
print_SOI (formatLine (Con a) a)
return a
eval_SOI (Add t u) = do a - eval_SOI t
b - eval_SOI u
incSOIstate
print_SOI (formatLine (Add t u) (a + b))
if (a + b) == 42
then raise (a+b) = The Ultimate Answer!!
else return (a + b)
runEval exp = case eval_SOI exp of
Raise a - a 0
SOIE p - let (result, state, output) = p 0 in
(result,state,output) --Result = ++ show result
++ Recursions = ++ show state ++ Output = ++ output
--runEval (Add (Con 10) (Add (Con 28) (Add (Con 40) (Con 2
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Re: [Haskell-cafe] state and exception or types again...
Il Mon, Aug 28, 2006 at 08:23:15PM +0200, Andrea Rossato ebbe a scrivere:
The previous code was not complete, and so testable.
at the end there is the output.
there it is:
module Monads where
data Term = Con Int
| Add Term Term
deriving (Show)
type State = Int
type Output = String
formatLine :: Term - Int - Output
formatLine t a = eval ( ++ show t ++ ) = ++ show a ++ -
data Eval_SOI a = Raise { unPackMSOIandRun :: State - (a, State, Output) }
| SOIE { unPackMSOIandRun :: State - (a, State, Output) }
instance Monad Eval_SOI where
return a = SOIE (\s - (a, s, ))
m = f = SOIE (\x -
let (a, y, s1) = unPackMSOIandRun m x in
case f a of
SOIE nextRun - let (b, z, s2) = nextRun y in
(b, z, s1 ++ s2)
Raise e1 - e1 y --only this happens
)
-- (=) m f = case m of
--Raise e - error ciao -- why this is not going to
happen?
--SOIE a - SOIE (\x -
--let (a, y, s1) = unPackMSOIandRun m x in
--let (b, z, s2) = unPackMSOIandRun (f a) y
in
--(b, z, s1 ++ s2))
incSOIstate :: Eval_SOI ()
incSOIstate = SOIE (\s - ((), s + 1, ))
print_SOI :: Output - Eval_SOI ()
print_SOI x = SOIE (\s - ((),s, x))
raise x e = Raise (\s - (x,s,e))
eval_SOI :: Term - Eval_SOI Int
eval_SOI (Con a) = do incSOIstate
print_SOI (formatLine (Con a) a)
return a
eval_SOI (Add t u) = do a - eval_SOI t
b - eval_SOI u
incSOIstate
print_SOI (formatLine (Add t u) (a + b))
if (a + b) == 42
then raise (a+b) = The Ultimate Answer!!
else return (a + b)
runEval exp = case eval_SOI exp of
Raise a - a 0
SOIE p - let (result, state, output) = p 0 in
(result,state,output) --Result = ++ show result
++ Recursions = ++ show state ++ Output = ++ output
--runEval (Add (Con 10) (Add (Con 28) (Add (Con 40) (Con 2
will produce
(80,7,eval (Con 10) = 10 - eval (Con 28) = 28 - eval (Con 40) = 40 - eval
(Con 2) = 2 - = The Ultimate Answer!!eval (Add (Con 28) (Add (Con 40) (Con
2))) = 70 - eval (Add (Con 10) (Add (Con 28) (Add (Con 40) (Con 2 = 80 -
)
thats is:
eval (Con 10) = 10 -
eval (Con 28) = 28 -
eval (Con 40) = 40 -
eval (Con 2) = 2 - = The Ultimate Answer!!
eval (Add (Con 28) (Add (Con 40) (Con 2))) = 70 -
eval (Add (Con 10) (Add (Con 28) (Add (Con 40) (Con 2 = 80 -
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Re: [Haskell-cafe] state and exception or types again...
Andrea Rossato wrote:
Now I'm trying to create a statefull evaluator, with output and
exception, but I'm facing a problem I seem not to be able to
conceptually solve.
data Eval_SOI a = Raise { unPackMSOIandRun :: State - (a, State,
Output) } | SOIE { unPackMSOIandRun :: State - (a,
State, Output) }
If you instead consider that you want the wrapped function to evaluate to a
result that allows you to continue, or a result that is an exception, then
you could use a flag to record whether or not an exception occurred eg:
data Eval_SOI a = SOIE {runSOIE :: State - (a, State, Output, Bool)}
where the 4th element of the tuple is True iff we can continue or False iff
an exception occurred.
raise x e = Raise (\s - (x,s,e))
raise x e = SOIE {\s - (x, s, e, False)}
instance Monad Eval_SOI where
return a = SOIE (\s - (a, s, , True))
m = f = SOIE $ \x -
let
r1@(a, y, o1, ok1) = runSOIE m x
in if ok1
then
let
(b, z, o2, ok2) = runSOIE (f a) y
in (b, z, o1 ++ o2, ok2)
else r1
ie if runSOIE m x does not result in an exception then we continue with the
second computation otherwise we just return the exception (r1) immediately.
Note I have not tested the above code so it may have a bug in it (also I
renamed s1 and s2 to o1 and o2 to avoid confusion with the use of s to
mean state)...
Regards, Brian.
--
Logic empowers us and Love gives us purpose.
Yet still phantoms restless for eras long past,
congealed in the present in unthought forms,
strive mightily unseen to destroy us.
http://www.metamilk.com
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Re: [Haskell-cafe] state and exception or types again...
Il Mon, Aug 28, 2006 at 09:28:02PM +0100, Brian Hulley ebbe a scrivere:
data Eval_SOI a = SOIE {runSOIE :: State - (a, State, Output, Bool)}
seems simple and neat...
else r1
ie if runSOIE m x does not result in an exception then we continue with the
second computation otherwise we just return the exception (r1) immediately.
well, I thought that this was not possible:
(=) :: m a - (a - m b) - m b
This is why I think that two constructors are needed, but with two
constructors is not possible...;-)
I'm trying to dig into this problem also to see if it has to do with
monad laws. But probably this is just a late night guess.
Thanks for your kind attention.
Best regards,
Andrea
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Re: [Haskell-cafe] state and exception or types again...
Il Mon, Aug 28, 2006 at 09:28:02PM +0100, Brian Hulley ebbe a scrivere: where the 4th element of the tuple is True iff we can continue or False iff an exception occurred. I'm starting to believe that the best method is just take the way StateT takes... without reinventing the wheel... Regards, Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] state and exception or types again...
Andrea Rossato wrote: Now I'm trying to create a statefull evaluator, with output and exception, but I'm facing a problem I seem not to be able to conceptually solve. If a computation fails in your monad, do you still want to return a value of the result type? I'd expect not, and hence remove the 'a' from the 'Raise' constructor. data Eval_SOI a = Raise (State -(State, Output)) | SOIE (State - (a, State, Output)) The above is very similar to using the monad foundation classes: import Control.Monad.Error import Control.Monad.State hiding (State) import Control.Monad.Writer type Eval_SOI = ErrorT String (StateT State (Writer Output)) ...assuming that you're happy to send back a descriptive String when a computation fails. Have a look at the Control.Monad.Error source code, to see how 'instance Error a = Monad (Either a)' is defined. It's the sort of thing you were trying to do in your 'instance Monad Eval_SOI'. Regards, Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
