I'm new to haskell and have started it because I'm doing it at UNI, I've had previous experience in C, C# and HTML, CSS. We had a lab this week where we were given the code :
data Token = Plus | Minus | Times | Divided_By | Power | Num {number :: Double}
deriving (Show, Eq)
type Expression = [Token]
tokenizer :: String -> Expression
tokenizer expression_as_string = case expression_as_string of
"" -> []
c: cs -> case c of
'+' -> Plus : tokenizer cs
'-' -> Minus : tokenizer cs
'*' -> Times : tokenizer cs
'/' -> Divided_By : tokenizer cs
'^' -> Power : tokenizer cs
_ | c `elem` ['0' .. '9'] -> case reads expression_as_string of
[(value, rest)] -> Num value : tokenizer rest
_ -> error "Could not read number"
| c `elem` [' ', '\t'] -> tokenizer cs
| otherwise -> error "Unknown Symbol"
expression_to_string :: Expression -> String
expression_to_string expr = case expr of
[] -> ""
e: es -> case e of
Plus -> " + " ++ expression_to_string es
Minus -> " - " ++ expression_to_string es
Times -> " * " ++ expression_to_string es
Divided_By -> " / " ++ expression_to_string es
Power -> " ^ " ++ expression_to_string es
Num x -> (show x) ++ expression_to_string es
eval_string_expression :: String -> String
eval_string_expression s = expression_to_string (eval (tokenizer s))
So now we are meant to implement out own function called eval which should solve this equation, when typed in ghci:
eval [Num 3.2, Plus, Minus, Num 4.2, Minus, Num 5.3, Plus, Num 6.3] [Num {number = 0.0}]
So at this point i've used pattern matching (judging that i understand it) and got this far:
eval :: Expression -> Expression
eval list = case list of
[] -> []
[Num x, op, Num y] -> case op of
Plus -> [Num (x + y)]
Minus -> [Num (x - y)]
Times -> [Num (x * y)]
Divided_By -> [Num (x / y)]
Power -> [Num (x ** y)]
_ -> error "......"
Now if i type this into ghci: eval[Num 3.1, Plus, Num 4.2] it returns 7.300000000001 because it is a double, now im not sure if this is correct. Though i do know that this is non-exhaustive because i havent taken care of all the possibilities. What i need help with is taking a negative number into consideration such as Minus, Plus which should return a Minus but I also need to take note of how order works, such as Powers, then Multiplication/Division then Addition/Subtraction. Our lecturer has given us the return value if the eval function is called: eval ((eval [Num x, op, Num y]) ++ op_next: remaining_expr)
So if anyone can help, if you understand what i am saying it would be helpful, ohh and go easy please im still trying to learn! :) This lab is meant to teach us pattern matching but i still don't fully understand it.
I probably should note that I know what I am meant to do but just not how to write it correctly!
