[Haskell-cafe] Mutable arrays

Jonathan Cast jonathanccast at fastmail.fm
Wed Feb 6 21:16:24 EST 2008

On 6 Feb 2008, at 5:17 PM, Jeff φ wrote:

> IO(U)Arrays are only one variant of mutable Array, there are also ST 
> (U)Arrays,
> which are often preferred.
> I should have worded my question better.  The MArray interface is  
> implemented in both the ST and IO monad.  A state monad seems like  
> a logical place for mutable arrays.  However, I don't understand  
> the motivation for implementing it in IO.  Were mutable arrays  
> added to IO because it would be difficult to write code that does  
> both IO and manipulates arrays otherwise?


As I understand it, the reasons are roughly as follows:

(a) The ST monad is a fairly clever hack, and is some years (4 or 5?)  
younger than IO.
(b) You can't have an ST-mutable array as a global variable, but you  
can do this with IO.  It's ugly, but some libraries are rumored to  
require it (see the discussion in http://haskell.org/haskellwiki/ 
(c) In particular, libraries that need to do I/O and/or FFI are  
rumored to be particularly in need of top-level mutable state, so  
it's natural to combine the two (or three).
(d) It's very difficult to combine monads.  Clean's uniqueness types / 
can/ be combined (as could arbitrary state monads).  In general, IMHO  
what you normally want when combining monads is their coproduct.  A  
general coproduct is quite ugly, but it simplifies nicely in  
particular cases.  Clean, by restricting the problem to combining  
uniqueness-type-based state monads, can combine monads more easily  
than Haskell can in the general case.  15 years ago, when these  
decisions were made, it seemed easier to have a single monad.
(e) To a certain extent, IO is the monad in Haskell for `everything  
other languages can do that we can't'.  That's just the nature of the  
beast; the name IO is simply taken from the most prominent example.   
So IO's mandate is to do everything C can do better.
(f) Some of use consider the IO monad (and mutable arrays in general)  
deprecated for precisely this reason; the rest seem to want C with a  
nicer syntax.  They use IO, we try to avoid it entirely.


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