[Haskell-cafe] state and exception or types again...

[Brian Hulley]

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.

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