Question

I see that Seq has a cast function from IEnumerable to Seq, but how do I get it to work?

open System.Text.RegularExpressions;;
let v = Regex.Match("abcd", "(ab)");;
Seq.cast (v.Captures);;

This produces,

error FS0030: Value restriction. The value ‘it’ has been inferred to have generic type
val it : seq<‘_a>
Either define ‘it’ as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.

Answer 1

Be explicit about the type:

Seq.cast<Match> (v.Captures)

Otherwise cast needs context to infer the right return type, and on a line by itself like there, there is no such context for type inference to use.

(This line converts the non-generic IEnumerable into a generic IEnumerable<Match> a.k.a. seq<Match>.)