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Pattern matching in function arguments
f#

I noticed that I can do something like this:

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type Vector = 
    ThreeVector of int*int*int

let AddFirstTwo (ThreeVector(first,second,_)) = first+second

Couple of questions about this:

1) Why must ThreeVector(first,second,_) be in parentheses? If I don't include the parens it fails to compile.

2) I've been trying to guess the syntax so that I can have multiple patterns similar to what I'd have in a match statement, but I've been unsuccessful. For example, the following compiles but gives a warning:

> let AddFirstTwo (ThreeVector(first,second,7)) = first+second
warning FS0025: Incomplete pattern matches on this expression. For example, the value 'ThreeVector(_, _, 0) will not be matched.

But nevertheless I can call this function, and as long as the third coordinate is 7 it will work fine. Is there a syntax so that I can complete this pattern? I know that it's semantically equivalent to just saying 

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let AddFirstTwo x = 
    match x with
        ThreeVector(first,second,7) -> first+second

and using that syntax I can easily complete the pattern. But I like the succintness of the first method, for example not having to declare the variable x, since I am only interested in one of its components, and not it as a whole.

.

You need the parens, lest it parse like

let AddFirstTwo ThreeVector (x,y,z) =

e.g. where AddFirstTwo takes two parameters, the first being named 'ThreeVector', and the second being a tuple. The parens change the interpretation to a single-argument that is a pattern. This is also true for binding values, a la

let (ThreeVector(x,y,z)) = expr

without the parens, you define a new function. See also 6.7.1.2

[link:research.microsoft.com]

You can't specify multiple alternatives in a function declaration; you must use 'match', or 'function' as in

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#light

type Vector = 
    ThreeVector of int*int*int

let AddFirstTwo = function
    | ThreeVector(first,second,7) -> first+second
    | ThreeVector(first,second,third) -> 0

see also 6.6.4

[link:research.microsoft.com]

By on 12/6/2008 1:59 PM ()

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