[Haskell-cafe] Merry monad mixup?
The first and third work, but not the second. Why?
Michael
==
f :: String - IO ()
f s = do putStrLn s
{-
g :: [String] - IO ()
g l = do s - l
putStrLn s
-}
{-
h :: [Int] - [Int]
h l = do i - l
return (i+1)
-}
==
serialize2.hs:29:9:
Couldn't match expected type `[String]'
against inferred type `IO String'
In a stmt of a 'do' expression: s - l
In the expression:
do { s - l;
putStrLn s }
In the definition of `g':
g l = do { s - l;
putStrLn s }
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Re: [Haskell-cafe] Merry monad mixup?
On Fri, 2011-01-28 at 11:20 -0800, michael rice wrote: The first and third work, but not the second. Why? When you use a do block, it can be the syntactic sugar for whatever monad you like; but you do have to make a choice. Your first example had a do block for the IO monad. Your third example used the [] monad. Both are fine. The second, though, wasn't clear on what monad it was using. When you used the (-) syntax to nondeterministically choose from a list, the compiler settled upon the [] monad. But then the next line was a statement in the IO monad. That's inconsistent, hence the error. Perhaps you wanted to build a monad out of both behaviors? In this case, you should likely look into monad transformers, and in particular, the ListT monad transformer in the List package. This would allow you to write the code you did in a monad called ListT IO, except that the IO actions would need to be lifted through the use of either `lift` or `liftIO`. -- Chris Smith ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Merry monad mixup?
Beware of ListT. It only works if your internal monad is commutative, which IO is not. (Maybe would work, for example) On Fri, Jan 28, 2011 at 2:41 PM, Chris Smith cdsm...@gmail.com wrote: On Fri, 2011-01-28 at 11:20 -0800, michael rice wrote: The first and third work, but not the second. Why? When you use a do block, it can be the syntactic sugar for whatever monad you like; but you do have to make a choice. Your first example had a do block for the IO monad. Your third example used the [] monad. Both are fine. The second, though, wasn't clear on what monad it was using. When you used the (-) syntax to nondeterministically choose from a list, the compiler settled upon the [] monad. But then the next line was a statement in the IO monad. That's inconsistent, hence the error. Perhaps you wanted to build a monad out of both behaviors? In this case, you should likely look into monad transformers, and in particular, the ListT monad transformer in the List package. This would allow you to write the code you did in a monad called ListT IO, except that the IO actions would need to be lifted through the use of either `lift` or `liftIO`. -- Chris Smith ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Merry monad mixup?
And by works, I mean, ListT is is a monad only if the internal monad is commutative. On Fri, Jan 28, 2011 at 2:46 PM, Daniel Peebles pumpkin...@gmail.comwrote: Beware of ListT. It only works if your internal monad is commutative, which IO is not. (Maybe would work, for example) On Fri, Jan 28, 2011 at 2:41 PM, Chris Smith cdsm...@gmail.com wrote: On Fri, 2011-01-28 at 11:20 -0800, michael rice wrote: The first and third work, but not the second. Why? When you use a do block, it can be the syntactic sugar for whatever monad you like; but you do have to make a choice. Your first example had a do block for the IO monad. Your third example used the [] monad. Both are fine. The second, though, wasn't clear on what monad it was using. When you used the (-) syntax to nondeterministically choose from a list, the compiler settled upon the [] monad. But then the next line was a statement in the IO monad. That's inconsistent, hence the error. Perhaps you wanted to build a monad out of both behaviors? In this case, you should likely look into monad transformers, and in particular, the ListT monad transformer in the List package. This would allow you to write the code you did in a monad called ListT IO, except that the IO actions would need to be lifted through the use of either `lift` or `liftIO`. -- Chris Smith ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Merry monad mixup?
So, my suspicions are confirmed. Thanks, all. Michael --- On Fri, 1/28/11, Daniel Peebles pumpkin...@gmail.com wrote: From: Daniel Peebles pumpkin...@gmail.com Subject: Re: [Haskell-cafe] Merry monad mixup? To: Chris Smith cdsm...@gmail.com Cc: michael rice nowg...@yahoo.com, haskell cafe haskell-cafe@haskell.org Date: Friday, January 28, 2011, 2:47 PM And by works, I mean, ListT is is a monad only if the internal monad is commutative. On Fri, Jan 28, 2011 at 2:46 PM, Daniel Peebles pumpkin...@gmail.com wrote: Beware of ListT. It only works if your internal monad is commutative, which IO is not. (Maybe would work, for example) On Fri, Jan 28, 2011 at 2:41 PM, Chris Smith cdsm...@gmail.com wrote: On Fri, 2011-01-28 at 11:20 -0800, michael rice wrote: The first and third work, but not the second. Why? When you use a do block, it can be the syntactic sugar for whatever monad you like; but you do have to make a choice. Your first example had a do block for the IO monad. Your third example used the [] monad. Both are fine. The second, though, wasn't clear on what monad it was using. When you used the (-) syntax to nondeterministically choose from a list, the compiler settled upon the [] monad. But then the next line was a statement in the IO monad. That's inconsistent, hence the error. Perhaps you wanted to build a monad out of both behaviors? In this case, you should likely look into monad transformers, and in particular, the ListT monad transformer in the List package. This would allow you to write the code you did in a monad called ListT IO, except that the IO actions would need to be lifted through the use of either `lift` or `liftIO`. -- Chris Smith ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
