Random questions after a long haskell coding day

Mark P Jones mpj@cse.ogi.edu
Tue, 29 Jan 2002 08:43:47 -0800

| >> Any thumb rule for using arrays? I'm expecting access to be 
| >> O(1), it is right?
| > In GHC, yes.
| (Shouldn't this really be required?  I mean, the whole *point* of using
| arrays is to have O(1) random access, isn't it?)

In Hugs, no.  In an ideal world, the answer would be yes, but the
internal data structures that Hugs uses make it difficult to support
variable size structures.  So everything is coded up in terms of
pairs.  (Read the Gofer implementation report for more details and
insight.)  There was a time when Hugs included support for O(1)
random access  by allocating arrays in a separate heap (called the
`flat resource' for any Hugs historians out there).  But that code
was removed in preparation for the grand merger of Hugs and GHC (at
which point Hugs was to have inherited the runtime system of GHC,
complete with flat arrays).  But alas, that project faltered, the
integrated system never made it past a "thrill-seekers" release,
and Hugs today is without O(1) access to array components.  For most
purposes, and in the context of an interpreter, it really shouldn't
matter too much; if you're looking for meaningful performance
measurements, then you should really be using a compiler that doesn't
incur the interpretive overhead of Hugs.

| Can we also rely on destructive updates for the monadic arrays?

On this point, in Hugs, I believe the answer is yes.

All the best,