Hi,
the problem is that the function m in your code expects a parameter of type seq<int> and the code gives it a value of type LazyList<int>. The LazyList is of course compatible with seq type, but F# doesn't automatically insert the coertion.

You can either add the coertion explicitly:

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let n = m (L :> seq<int>)

Or modify the function m, so it accepts any type that implements the seq<int> interface:

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// parameter is any type that is a subtype of seq<int>
let m (L:'a when 'a :> seq<int>) = Seq.iter (fun x -> ()) L

// The same thing can be written using the '#' shortcut:
let m (L:#seq<int>) = Seq.iter (fun x -> ()) L