CODEWITHSUNDEEP
f#
Russell
Russell

Reputation: 599

Pattern matching on a list of tuples

I am attempting to create a function to pattern match a structure which is a tuple containing a value and a list of tuples of the form

'a * ('b * 'c) list -> 'b list

for example given the following:

let clubDetails = ("MyClub", [("Secretary", "Jill");("Captain", "Bob");("Email", "[email protected]")])

I need a function to return the list ["Secretary";"Captain";"Email"]

So I thought I could do something like this :

let getClubAttributes ca = 
        match ca with
        | (a, [(b,c)]) -> [b]
        | _ -> []

but here 

getClubAttributes clubDetails

Returns the empty list. I feel like I am missing something pretty obvious.

Thanks, Russell.

Upvotes: 2

Views: 402

Answers (1)

TheInnerLight
TheInnerLight

Reputation: 12184

The pattern match form that you have written will only match if the list in your tuple has exactly one element.

I note that you have written a default _ case in your pattern match which returns the empty list and this is the case you are hitting. I suspect you've added this case to remove the compiler warning but the compiler warning is, in fact, warning you of this exact problem.

You do not actually need a pattern match because your data structure can be handled with only one case.

let clubDetails = ("MyClub", [("Secretary", "Jill");("Captain", "Bob");("Email", "[email protected]")])

let getClubAttributes (ca, attribList) =
    List.map fst attribList

The new getClubAttributes function simply creates a new list from the original by taking the first element of each item in the list.

Upvotes: 5

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