Question

I suspect that I am missing something very obvious here but this doesn’t work:

let t = Array2D.create 1 1 1.0
for x in t do printfn "%f" x;;

It fails with

error FS0001: The type ‘obj’ is not compatible with any of the types float,float32,decimal, arising from the use of a printf-style format string

Interestingly using printf "%A" or "%O" prints the expected values which suggests to me that the problem is with the type inference

The corresponding code for a 1D array works fine

let t = Array.create 1 1.0
for x in t do printfn "%f" x;;

For reference this is on version 2.0 (both interactive and compiler) running on the latest mono

Answer 1

In .NET, a 1D array implicitly implements IList, which means it also implements (by inheritance) IEnumerable<T>. So, when you run:

let t = Array.create 1 1.0
for x in t do printfn "%f" x;;

the F# compiler emits code which gets an implementation of IEnumerable<T> (seq<T> in F#) from t, then iterates over it. Since it’s able to get an IEnumerable<T> from the array, x will have type T.

On the other hand, multi-dimensional arrays (2d, 3d, etc.) only implement IEnumerable (not IEnumerable<T>) so the F# compiler infers the type of x as System.Object (or obj, in F#).

There are two solutions for what you want:

Cast each individual value within the loop, before printing it:

for x in t do printfn "%f" (x :?> float);;

Or, use Seq.cast to create and iterate over a strongly-typed enumerator:

for x in (Seq.cast<float> t) do printfn "%f" x;;