Hi Dan,
You have written a great explanation of how
ListT works by writing out its definitions in an
interesting way!
Dan Piponi wrote:
A slightly different approach that doesn't use anything unsafe:
A list of type [Char] is essentially a
solution to the equation
X = Maybe (Char,X)
Yes. In fact, the type
data Y a = Y (Maybe (a, Y a))
is exactly equivalent to the type [a].
...to intersperse IO along the computation of the list...
define
data X = X { unX :: IO (Maybe (Char,X)) }
So that is exactly equivalent to the type ListT IO Char.
test = X $ do
a <- getChar
if a=='q'
then return Nothing
else return (Just (a,test))
That translates to:
test = do
a <- liftIO getChar
guard $ a /= 'q'
return $ a `mplus` test
test2 :: X -> IO ()
test2 test = do
a <- unX test
case a of
Nothing -> return ()
Just (a,b) -> do
print a
test2 b
Translation:
test2 = runListT . mapM (liftIO print)
Regards,
Yitz
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