F# Seq.head & Seq.tail type mismatch with custom types

Question

type Suit = Spades | Clubs | Hearts | Diamonds
type Rank = Ace | Two | Three | Four | Five | Six | Seven | Eight | Nine | Ten | Jack | Queen | King
type Card = {suit: Su         


        

Answer 1

You're passing cardValue the Seq.head function; you need to apply that function first and pass the result instead:

let rec handValue (hand:Hand) =
    if Seq.length hand = 1
    then cardValue (Seq.head hand)
    else cardValue (Seq.head hand) + handValue (Seq.tail hand)

N.b. this will fail if passed an empty sequence and isn't tail recursive; fixing these, along with a little redundancy removal, yields something like:

let handValue hand =
    let rec impl acc h =
        if Seq.isEmpty h then acc
        else impl (cardValue (Seq.head h) + acc) (Seq.tail h)
    impl 0 hand

That said, this is extremely inefficient; this code structure would work fine when using a list rather than a seq, but you're much better off using higher-order functions to process the latter:

let handValue hand = hand |> Seq.sumBy cardValue