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Pattern matching using if statement instead of "match x with"
f#

Hi guys... just wondering if there might be a way to pattern match using an if statement... Say I have an option union type and I want to check the following:

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let x = None

if (x = Some(_)) then .... elif (x = None) ....

Also, another question I have is how can I unwrap a variable such as in the Some(..) case above, is there a way to unwrap the outer Some and obtain the inner value it holds?

WOuld it perhaps be:

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let Some(y) = x

And what if x = None. Then would that above line return an error?

.

Pattern matching (with "match with") is the usual way to do.
The Option module has many useful functions you might want to use. To unwrap a value:

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let x = Some 2


Option.get x

To test an option, you can use "x.IsNone", "x.IsSome", or "x = None".
For other sum types, you might need to write your own functions or extension methods.

Laurent.

By on 3/31/2011 5:49 PM ()

Why do you need this?

By on 3/31/2011 7:42 AM ()

Hi,

for the first part I would do the same thing that is avaiable for F#'s Option-type. Implement members IsSome/IsNone or HasValue or whatever and maybe add static-checkers as well.

For the second part: yes in this case you'll get an warning and an error at runtime (just try with F#-interactive:

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let (Some y) = x;;

----^^^^^^^^

stdin(3,5): warning FS0025: Incomplete pattern matches on this expression. For example, the value 'None' may indicate a case not covered by the pattern(s).

Microsoft.FSharp.Core.MatchFailureException: The match cases were incomplete

at <StartupCode$FSI_0003>.$FSI_0003.main@()
By on 3/22/2011 11:22 PM ()

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