What does a.{X} mean in OCaml?

Question

I'm currently trying to port some OCaml to F#. I'm "in at the deep end" with OCaml and my F# is a bit rusty.

Anyway, the OCaml code builds fine in the OCaml compiler, but (not surprisingly) gives a load of errors in the F# compiler even with ML compatibility switched on. Some of the errors look to be reserved words, but the bulk of the errors are complaining about the .{ in lines such as:

 m.(a).(b) <- w.{a + b * c};

a,b,c are integers.

I've done a lot of searching through OCaml websites, Stackoverflow, the English translation of the French O'Reilly book, etc. and cannot find anything like this. Of course it doesn't help that most search facilities have problems with punctuation characters! Yes I've found references to . being used to refer to record members, and { } being used to define records, but both together? From the usage, I assume it is some kind of associative or sparse array?

What does this syntax mean? What is the closest F# equivalent?

Answer 1

There is a pdf of the oCaml documentation/manual available here:

http://caml.inria.fr/distrib/ocaml-3.12/ocaml-3.12-refman.pdf

On page 496 (toward the bottom of the page), it says of generic arrays and their get method:

val get : (’a, ’b, ’c) t -> int array -> ’a

Read an element of a generic big array. Genarray.get a [|i1; ...; iN|] returns the element of a whose coordinates are i1 in the first dimension, i2 in the second dimension, . . ., iN in the N-th dimension.

If a has C layout, the coordinates must be greater or equal than 0 and strictly less than the corresponding dimensions of a. If a has Fortran layout, the coordinates must be greater or equal than 1 and less or equal than the corresponding dimensions of a. Raise Invalid_argument if the array a does not have exactly N dimensions, or if the coordinates are outside the array bounds.

If N > 3, alternate syntax is provided: you can write a.{i1, i2, ..., iN} instead of Genarray.get a [|i1; ...; iN|]. (The syntax a.{...} with one, two or three coordinates is reserved for accessing one-, two- and three-dimensional arrays as described below.)

Further, it says (specifically about one dimensional arrays):

val get : (’a, ’b, ’c) t -> int -> ’a

Array1.get a x, or alternatively a.{x}, returns the element of a at index x. x must be greater or equal than 0 and strictly less than Array1.dim a if a has C layout. If a has Fortran layout, x must be greater or equal than 1 and less or equal than Array1.dim a. Otherwise, Invalid_argument is raised.

In F#, you can access array elements using the Array.get method as well. But, a closer syntax would be w.[a + b * c]. In short, in F#, use [] instead of {}.