11 Jan 2002 17:10:16 -0500
Theodore Norvell <firstname.lastname@example.org> writes:
> Jorge's question raised a question in my mind. The IOExts
> module has many of the same features as the ST module, why
> are there two ways to do the same thing? Is the ST module
> only there for legacy purposes?
My user view is that I appreciate the presence of both.
When I write a pure function that can be implemented efficiently in
imperative programming, I want the ST monad to support mutable
variables. E.g., to implement "f n returns a list of all primes
between 2 and n", I want to use a mutable array and hide it.
When I write an I/O-bound routine that can be implemented conveniently
with state variables, I want the IO monad to support mutable
Now my grief is that I cannot write a subprogram with state variables
and have it reused in ST and IO. Fortunately I can write a subprogram
with mutable arrays and have it reused in ST and IO, so I can write
"sort a given array"; but I cannot write "increment a given integer
Of course, you can tell me to use mutable arrays of length 1 to
simulate mutable variables. Fine!