On 12/7/06, Cat Dancer <[EMAIL PROTECTED]> wrote:
On 12/7/06, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
I'm sure from a single example I could understand what was going on
and elaborate from there.
Let's say I want to get a line from the user, and either return an
integer or an error string using ErrorT.
import Control.Monad.Error
import Control.Monad.Trans
foo :: ??
foo :: ErrorT String IO Int
if you're going to use this very often, you can use a line like
type M = ErrorT String IO
and then foo :: M Int
foo = do -- something like this?
a <- getLine
Since ErrorT String IO Int is not the same as IO, you can't use IO
operations directly. In this case, you want:
< a <- lift getLine
if length a == 1
then return 123
else throwError "not a single character"
This is all fine
main = do
r <- ?? foo ??
You want:
< r <- runErrorT foo
and then this will behave as expected:
print r -- prints Left "not a single character" or Right 123 ?
Your foo operation is in a monad which wraps IO. lift allows IO
operations inside that wrapper, and runErrorT removes the wrapper.
The same basic pattern works for other wrappers (like StateT or ListT)
or combinations of wrappers (like StateT Int (ErrorT String IO))
/g
--
It is myself I have never met, whose face is pasted on the underside of my mind.
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