F# pattern-matching gone wrong - f#

Question

I am just starting out with F# so this might be a trivial question but I am not able to understand why the pattern matching in my code acts as it does.
Quick explanation of the code:
The func calcNextMatch should recurse a list and if 2 elements are equal they should be added together.
In the end the func should return a number that is the addition of all digits that has a match with the next digit in the list.
f.ex. [1;3;2;2;5] should return 4
Code:
let rec printList l =
match l with
| head :: tail -> printf "%d " head; printList tail
| [] -> printfn ""
let rec calcNextMatch list =
printList list
match list with
| [] -> 0
| _ :: tail ->
printList tail
let h = Seq.head list
let t = Seq.tryHead tail
printfn "h: %i" h
printfn "t: %O" t
match t with
| Some h ->
printfn "TAIL t: %i is equal to HEAD h: %i" t.Value h
printfn "Calculation is: %i" (t.Value + h)
(t.Value + h) + calcNextMatch tail
| _ -> calcNextMatch tail
let sequence = [ 1;3;2;2;5 ]
let run = calcNextMatch sequence
When I run this code the problem is that the pattern-matching
does not work as I expect it.
f.ex this print output from running the script.
h: 1
t: Some(3)
TAIL t: 3 is equal to HEAD h: 3
this means that F# has matched
match t with
| Some h ->
in a case where t = Some(3) and h = 1
which translates to
match 3 with
| Some 1 ->
and that I do not understand.
The print before the matching states the value of t and h to 3 and 1 but in the pattern-matching the value of h has change to 3
How is this possible?

Answer 1

You can only pattern match against constant literals, otherwise the value get bounded as if was a new let-binding.
In these cases what you do normally is to add a when condition:
match t with
| Some x when x = h ->
Also notice that you can use pattern match further to simplify your code, for instance here:
| _ :: tail ->
printList tail
let h = Seq.head list
You can write:
| h :: tail ->
printList tail
Also all this portion:
| _ :: tail ->
printList tail
let h = Seq.head list
let t = Seq.tryHead tail
printfn "h: %i" h
printfn "t: %O" t
match t with
| Some h ->
printfn "TAIL t: %i is equal to HEAD h: %i" t.Value h
printfn "Calculation is: %i" (t.Value + h)
(t.Value + h) + calcNextMatch tail
becomes:
| h :: tail ->
printList tail
//printfn "h: %i" h
//printfn "t: %O" t
match tail with
| t::_ when t = h ->
printfn "TAIL t: %i is equal to HEAD h: %i" t h
printfn "Calculation is: %i" (t + h)
(t + h) + calcNextMatch tail
And you can unify all matches in one, so your whole function becomes:
let rec calcNextMatch list =
printList list
match list with
| [] -> 0
| h::x::tail when x = h -> x + h + calcNextMatch (x::tail)
| _::tail -> calcNextMatch tail
Finally, when you're done with debugging, you can remove the prints and since the last parameter of your function is the one you match against, you can use the keyword function, also use an as pattern to avoid reconstructing the list:
let rec calcNextMatch = function
| [] -> 0
| h::((x::_) as tail) when x = h -> x + h + calcNextMatch tail
| _::tail -> calcNextMatch tail