Stymied by mutable arrays in the ST monad
I've been trying to use mutable arrays in the ST monad, and wrote out a little proof of concept function: idST :: [Int] -> [Int] idST xs = runST $ do array <- newListArray (1, (length xs)) xs return (getElems array) -- where idSt should be equivalent to id. And I get the error message: Couldn't match type `[Int]' with `Int' In the return type of a call of `getElems' In the first argument of `return', namely `(getElems array)' In a stmt of a 'do' block: return (getElems array) Obviously I'm making a very simple mistake here, but I can't seem to spot it. Can anyone offer some advice?
On 25 May 2012 18:50, Matthew Moppett
I've been trying to use mutable arrays in the ST monad, and wrote out a little proof of concept function:
idST :: [Int] -> [Int] idST xs = runST $ do array <- newListArray (1, (length xs)) xs return (getElems array)
-- where idSt should be equivalent to id.
And I get the error message:
Couldn't match type `[Int]' with `Int' In the return type of a call of `getElems' In the first argument of `return', namely `(getElems array)' In a stmt of a 'do' block: return (getElems array)
Obviously I'm making a very simple mistake here, but I can't seem to spot it. Can anyone offer some advice?
'getElems array' already has type 'ST s [Int]', you don't need another 'return'. Furthermore you need to help out with the type inference a little bit (it's similar to the read-show problem). This should work: idST :: [Int] -> [Int] idST xs = runST $ do array <- newListArray (1, (length xs)) xs :: ST s (STArray s Int Int) getElems array (You could also replace STArray by STUArray.)
Thanks, Tobias.
I guess my eyes kind of glazed over when I read "getElems :: (MArray a e
m, Ix i) => a i e -> m [e]" in the docs, and didn't relate that to the
meaning of "return"... lesson learnt.
About the extra type info needed -- what part of the type "ST s (STArray s
Int Int)" is the compiler unable to infer?
I've worked out from this that the error message "no instance for"... might
signal a missing type signature, but I'm having trouble working out the
general lesson of when the compiler needs some extra hints.
On 25 May 2012 18:50, Matthew Moppett
I've been trying to use mutable arrays in the ST monad, and wrote out a little proof of concept function:
idST :: [Int] -> [Int] idST xs = runST $ do array <- newListArray (1, (length xs)) xs return (getElems array)
-- where idSt should be equivalent to id.
And I get the error message:
Couldn't match type `[Int]' with `Int' In the return type of a call of `getElems' In the first argument of `return', namely `(getElems array)' In a stmt of a 'do' block: return (getElems array)
Obviously I'm making a very simple mistake here, but I can't seem to spot it. Can anyone offer some advice?
'getElems array' already has type 'ST s [Int]', you don't need another 'return'. Furthermore you need to help out with the type inference a little bit (it's similar to the read-show problem). This should work: idST :: [Int] -> [Int] idST xs = runST $ do array <- newListArray (1, (length xs)) xs :: ST s (STArray s Int Int) getElems array (You could also replace STArray by STUArray.)
participants (2)
-
Matthew Moppett -
Tobias Brandt