CODEWITHSUNDEEP
f#
Jamie Dixon
Jamie Dixon

Reputation: 4302

F# creating smaller tuple using Seq.map

I have written this F# code:

let tuples = [|("A",1,0);("A",2,3);("A",3,6)|]
let tupleSubset =
tuples
|> Seq.map(fun values -> 
    values.[0],
    values.[2])
printfn "%A" tupleSubset

I am getting: The operator 'expr.[idx]' has been used on an object of indeterminate type based on information prior to this program point. Consider adding further type constraints

Can anyone tell me what I am doing wrong?

Upvotes: 4

Views: 3076

Answers (2)

nicolas
nicolas

Reputation: 9825

The explanation of Shredderoy is correct, accessing an element by its index is reserved to arrays, which are contiguous blocks of memory.

For tuple, if you want to access the values of a tuple in F# the general method is to pattern match 

let tuple = a,b,c
let (x,y,z) = tuple

In the case of pair, the function fst and snd also give a similar access

PS : In you initial array, you dont need to use parenthesis, as the compiler recognizes the , as a tuple

Upvotes: 0

Shredderroy
Shredderroy

Reputation: 2920

A simple way around the error you are getting would be the following:

let tuples = [|("A",1,0); ("A",2,3); ("A",3,6)|]
let tupleSubset = tuples |> Seq.map (fun (a, b, c) -> a, b)
printfn "%A" tupleSubset

As to why you are getting the error: note that the kind of index dereferencing you are attempting with values.[0] and values.[1] works for arrays, dictionaries, etc., but not for tuples, whereas each values has the type string * int * int.

Since you don't need the third element of the tuple, you can even choose not to bind it to a symbol, by writing the second line as follows:

let tupleSubset = tuples |> Seq.map (fun (a, b, _) -> a, b)

Upvotes: 7

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