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sequence expression of the kind for pattern in sequence -> expression
f#book

I am reading the Expret F# book and it has the following in it:

The following is an example where the input is a
list of options. If the pattern fails to match the element of the enumerable, it is skipped, and no
result is yielded for that element:

> seq { for Some(nm) in [ Some("James"); None; Some("John") ] -> nm.Length };;
val it : seq<int> = [ 5; 4 ]

For some reason this throws MatchFailureException (obviously because None matches no pattern) instead of producing no value for the None entry. Did they change something in the language? How does this expression work now?

.

It seems something has changed. Interestingly, the following works like a charm:

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for Some(nm) in [ Some("James"); None; Some("John") ] do printfn "%d" nm.Length
By on 1/22/2009 7:30 AM ()

I hit the same problem in the same book. There is no errata on this (don't know where that would be submitted). It's now four builds back (1.9.2) from the current (1.9.6) version, maybe needs an update itself? Couldn't find a 1.9.2 version of the "informal" spec, so couldn't figure out anything there.

So, I verified that

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seq { for x in [Some("John"); None; Some("James")] do 

      match x with 

         | Some(nm) -> yield nm 

         | _ -> ()};;

works, but that seems a little verbose. How is everyone else dealing with this?

By on 6/10/2009 11:29 AM ()

For the book errata, see [link:www.expert-fsharp.com] (last update Jan 2009, we'll work on a new update today)

To submit new errata send direct to me (dsyme AT microsoft DOT com) or one of the other authors, or via the Apress site.

Thanks!

don

By on 6/12/2009 7:53 AM ()

Hmm. Strange that this works in a loop, but not in a sequence expression.
The way computation expressions are compiled did change between builds, so maybe that's what caused this.

You can get the desired result by using Seq.choose to grab only the Some(..) parts of the list, like this:

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let thisworks = seq { for nm in [ Some("James"); None; Some("John") ] |> Seq.choose id -> nm.Length }
By on 6/11/2009 12:49 AM ()

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