[Haskell] Re: ST/STRef vs. IO/IORef

Srinivas Nedunuri nedunuri at cs.utexas.edu
Wed Aug 3 11:43:45 EDT 2005

"Cale Gibbard" <cgibbard at gmail.com> wrote in message
news:89ca3d1f05080300353ecd7fd5 at mail.gmail.com...
Well, what to do with your computation depends on what that
computation is actually doing. Is the IO really critical to your
algorithm, or can it be done separately? Remember that pure
computation is lazy and so you can save yourself from interleaving a
lot of IO in with generating the data, if you are, for example,
printing it as it is computed.
<<Alas, I wish I could seperate the IO from the ST. Unf the IO in question
here is related to files and directories and the program I am writing (a
simple version control program) manipulates a mutable object graph (hence
the use of ST) and then interleaved with that needs to access File IO. runST
isn't much use to me since I don't want to run anything "locally" (I believe
that's what runST does - it manufactures an initial empty state and threads
that through). Rather everything operates on this global object model. >>

Basically, the distinction between ST and IO you should worry about is
the same as the one between pure values and IO. You don't want lots of
computations in the IO monad, because IO computations are harder to
reason about; it's harder to make guarantees about what they do.

ST computations should be thought of as computing pure functions,
similar to how ordinary Haskell functions would, except that they may
have multiple named state variables which are carried along during the
computation, and permit an imperative style of programming, for when
that is more natural. (Various graph algorithms, for instance.)
<< Can you elaborate on this a bit more? AFAIK, IO t  is just ST RealWorld
Since you never get to look at the state in an ST monad anyway, why does it
matter (for the purposes of reasoning about your prgram) whether its
RealWorld or some manufactured state?
e.g. I have a function "f data  = do x <- g data; y <- h x, etc." the only
information I have is that the given computations will happen in that order
given a suitable "state" value so referential transparency only extends as
far as being able to say that f data can be replaced by its right hand side.

As far as I can tell, stToIO shouldn't be needed in ordinary
programming. runST will serve quite well to extract a value from an ST

If what you are doing is fundamentally IO, and not just computing some
function, then you should write your computation in the IO monad. Note
that from the IO monad, you can hook up some pure computations (plain
functions, ST, etc.) to do whatever kind of processing is needed to
get from input to output. For this reason, when writing code in the IO
monad, one should only really have to worry about the input and output
to be performed.

I hope this is useful,
- Cale

On 02/08/05, Srinivas Nedunuri <nedunuri at cs.utexas.edu> wrote:
> Hello, I have some code that manipulates STRefs within the ST monad. All
> good and fine, until I come across some computation that uses lets say IO
> and everything skids to a halt. At this point I have 3 choices:
> 1. Define a ST State Transformer monad and do all my previous ST
> computations in that
> 2. convert all subsequent ST computations into IO computations using
> 3. stop using the ST monad and do everything in the IO monad
> I was wondering what advice folks had. In particular, what are the
> disadvantages to doing everything in the IO monad - ie why even bother
> the ST monad?
> Any help appreciated
> cheers
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