How to fix this bug in a math expression evaluator - f#

Question

I've written a typical evaluator for simple math expressions (arithmetic with some custom functions) in F#. While it seems to be working correctly, some expressions don't evaluate as expected, for example, these work fine:
eval "5+2" --> 7
eval "sqrt(25)^2" --> 25
eval "1/(sqrt(4))" --> 0.5
eval "1/(2^2+2)" --> 1/6 ~ 0.1666...
but these don't:
eval "1/(sqrt(4)+2)" --> evaluates to 1/sqrt(6) ~ 0.408...
eval "1/(sqrt 4 + 2)" --> will also evaluate to 1/sqrt(6)
eval "1/(-1+3)" --> evaluates to 1/(-4) ~ -0.25
the code works as follows, tokenization (string as input) -> to rev-polish-notation (RPN) -> evalRpn
I thought that the problem seems to occur somewhere with the unary functions (functions accepting one operator), these are the sqrt function and the negation (-) function. I don't really see what's going wrong in my code. Can someone maybe point out what I am missing here?
this is my implementation in F#
open System.Collections
open System.Collections.Generic
open System.Text.RegularExpressions
type Token =
| Num of float
| Plus
| Minus
| Star
| Hat
| Sqrt
| Slash
| Negative
| RParen
| LParen
let hasAny (list: Stack<'T>) =
list.Count <> 0
let tokenize (input:string) =
let tokens = new Stack<Token>()
let push tok = tokens.Push tok
let regex = new Regex(#"[0-9]+(\.+\d*)?|\+|\-|\*|\/|\^|\)|\(|pi|e|sqrt")
for x in regex.Matches(input.ToLower()) do
match x.Value with
| "+" -> push Plus
| "*" -> push Star
| "/" -> push Slash
| ")" -> push LParen
| "(" -> push RParen
| "^" -> push Hat
| "sqrt" -> push Sqrt
| "pi" -> push (Num System.Math.PI)
| "e" -> push (Num System.Math.E)
| "-" ->
if tokens |> hasAny then
match tokens.Peek() with
| LParen -> push Minus
| Num v -> push Minus
| _ -> push Negative
else
push Negative
| value -> push (Num (float value))
tokens.ToArray() |> Array.rev |> Array.toList
let isUnary = function
| Negative | Sqrt -> true
| _ -> false
let prec = function
| Hat -> 3
| Star | Slash -> 2
| Plus | Minus -> 1
| _ -> 0
let toRPN src =
let output = new ResizeArray<Token>()
let stack = new Stack<Token>()
let rec loop = function
| Num v::tokens ->
output.Add(Num v)
loop tokens
| RParen::tokens ->
stack.Push RParen
loop tokens
| LParen::tokens ->
while stack.Peek() <> RParen do
output.Add(stack.Pop())
stack.Pop() |> ignore // pop the "("
loop tokens
| op::tokens when op |> isUnary ->
stack.Push op
loop tokens
| op::tokens ->
if stack |> hasAny then
if prec(stack.Peek()) >= prec op then
output.Add(stack.Pop())
stack.Push op
loop tokens
| [] ->
output.AddRange(stack.ToArray())
output
(loop src).ToArray()
let (#) op tok =
match tok with
| Num v ->
match op with
| Sqrt -> Num (sqrt v)
| Negative -> Num (v * -1.0)
| _ -> failwith "input error"
| _ -> failwith "input error"
let (##) op toks =
match toks with
| Num v,Num u ->
match op with
| Plus -> Num(v + u)
| Minus -> Num(v - u)
| Star -> Num(v * u)
| Slash -> Num(u / v)
| Hat -> Num(u ** v)
| _ -> failwith "input error"
| _ -> failwith "inpur error"
let evalRPN src =
let stack = new Stack<Token>()
let rec loop = function
| Num v::tokens ->
stack.Push(Num v)
loop tokens
| op::tokens when op |> isUnary ->
let result = op # stack.Pop()
stack.Push result
loop tokens
| op::tokens ->
let result = op ## (stack.Pop(),stack.Pop())
stack.Push result
loop tokens
| [] -> stack
if loop src |> hasAny then
match stack.Pop() with
| Num v -> v
| _ -> failwith "input error"
else failwith "input error"
let eval input =
input |> (tokenize >> toRPN >> Array.toList >> evalRPN)

Answer 1

Before answering your specific question, did you notice you have another bug? Try eval "2-4" you get 2.0 instead of -2.0.
That's probably because along these lines:
match op with
| Plus -> Num(v + u)
| Minus -> Num(v - u)
| Star -> Num(v * u)
| Slash -> Num(u / v)
| Hat -> Num(u ** v)
u and v are swapped, in commutative operations you don't notice the difference, so just revert them to u -v.
Now regarding the bug you mentioned, the cause seems obvious to me, by looking at your code you missed the precedence of those unary operations:
let prec = function
| Hat -> 3
| Star | Slash -> 2
| Plus | Minus -> 1
| _ -> 0
I tried adding them this way:
let prec = function
| Negative -> 5
| Sqrt -> 4
| Hat -> 3
| Star | Slash -> 2
| Plus | Minus -> 1
| _ -> 0
And now it seems to be fine.

Answer 2

Edit: meh, seems I was late, Gustavo posted the answer while I was wondering about the parentheses. Oh well.
Unary operators have the wrong precedence. Add the primary case | a when isUnary a -> 4 to prec.
The names of LParen and RParen are consistently swapped throughout the code. ( maps to RParen and ) to LParen!
It runs all tests from the question properly for me, given the appropriate precedence, but I haven't checked the code for correctness.