In short: There was a change in the CTP version. It's "normal" that this example does not work anymore.Longer story:See [link:cs.hubfs.net] answer brilsmurf gave is a sequence iteration expression, while yours is a sequence expression. Apparantly these now react differently to match failures. Section 6.6.5 of the language spec ( [link:research.microsoft.com] ) describes the elaboration for sequence iteration expressions, but I didn't find such a thing for sequence expressions.
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I'm working my way through "Expert F#" and on page 60 I entered the following expression in F# interactive
seq { for Some(nm) in [Some("James"); None; Some("John")]->nm.Length};;
seq { for Some(nm) in [Some("James"); None; Some("John")]->nm.Length};;
----------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
stdin(19,23): warning FS0025: Incomplete pattern matches on this expression. For example, the value 'None' will not be matche
d
val it : seq<int>
= Error: Exception of type 'Microsoft.FSharp.Core.MatchFailureException' was thrown.
which according to the book I should get val it: seq<int> = [5;4]
Is this a bug in the code in the book?
Thanks,
Haroon