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Recursive values
f#

I've been trying to understand recursive values in F#. They make perfect sense in a lazy language like Haskell, but in a strict language like F#, I'm a bit stumped.

I tried:

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let rec xs = 1 :: ys


   

    and ys = 1 :: xs

and got:

error FS0191: Recursive values may not appear directly as a construction of the type 'List`1' within a recursive binding. This feature has been removed from the F# language. Consider using a record instead.

Fair enough. Then, I tried the following snippet in FSI:

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let rec f x = 1 + x + y


   

and y = f 1

(I know the code makes no sense)

and when I ask for the value of y, I get:

val it : int = 2

What's going on here?

.

Hi,

Mutually recursive values are forbidden, because it doesn't really make sens (which one is evaluated first?). A value and a function can be mutually recursive: this is somewhat dangerous, but it's sometimes useful (especially with laziness). I gave an example in a blog post: [link:laurent.le-brun.eu]

In your second example, the evaluation of y calls "f 1". "f 1" is 1 + x + y. x is 1, y is not yet initialized (I told you it's dangerous). For some reason, its value is 0 (this is the same as Unchecked.defaultof<int>).

When I read your message, I expected this result. However I thought you'd get a warning; it looks like a bug in F#, you should probably send a bug report about it.

Laurent

By on 4/2/2009 12:57 PM ()

The blog post contains a neat example of recursive values, thanks.

By on 4/2/2009 1:27 PM ()

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