RE: [Caml-list] Differences between Array and Strings

[David Allsopp <dra-news@metastack.com>]

Louis Jachiet wrote:
> Hi, I'am failing to understand the behavior of the code below. Why the
> string is not regenerated ? Why strings and array have not the same
> behavior ?

This has been commented on before (see the list archives) - if I understand correctly, it's a consequence of the code generator...

> > let l () = "1" ;;

Compiles to

(let (l/1030 (function param/1032 "1")) (makeblock 0 l/1030))

i.e. a reference to a constant with value "1" - not an allocation. So l always returns the same physical string, rather than allocating a new one. This makes sense if you're assuming that strings are by default used immutably (there have been plenty of discussions in the past on the relative merits of having the same string type for both immutable and mutable strings)

However, the constants clearly aren't used in a pool:

# let f () = "1";;
val f : unit -> string = <fun>
# let g () = "1";;
val g : unit -> string = <fun>

given:

# f () == g ();;
- : bool = false

> > (l ()).[0] <- '2';;
> > l ();;
> > l()==l();;
> 
> > let l () = [|1|] ;;
> > (l ()).(0) <- 2;;

This, however, compiles to

(let (l/1030 (function param/1032 (makearray 1))) (makeblock 0 l/1030))

i.e. it allocates a fresh array each time. The behaviour of the compiler would suggest that its authors considered string "constants" to be just that but the syntax for array literals to be simply sugar for an array allocation followed by initialisation of the members.

As a rule of thumb, if you're dealing with mutable strings, it's a good idea to be very clear who allocated them and where so personally I quite like the fact that the compiler forces you to need to write:

let l () = String.copy "1"

which will behave as you expected.

HTH,


David