Hi John
You need some more /machinery/ in your evaluator.
One possibility is to make it a monadic evaluator - you could then
choose to use IO and print things as you go, or a writer monad which
'collects' (think logging) as evaluation progresses. The at the end
you'd show both the log and the answer.
The writer monad has a run function, which you would need to run the
extended evaluator with
runWriter :: Writer w a -> (a, w)
The result of runWriter is a pair of a (the answer) and w (the log).
Writer is more general than just logging, but in this instance logging
is close to what you might want.
Best wishes
Stephen
2009/11/22 John Moore
Hi All,
I'm trying to work out how to show the steps in evaluating "How the function is calculated". In other words I want this program to actually print out the steps. For e.g. eval (Multiply(Add(Val 10) (Val 20)) (Val 3) Which is add 10+20 and multiply by 3 = 90
The program then produces( Prints out) something like
working: eval (Multiply(Add(Val 10) (Val 20)) (Val 3) working Add(Val10) (Val 20) answer 1 Val 30 answer 2 Val 90
This is what I have so far but it wont even do the calculations. Any help appreciated.
data Expression = Val Float | Add Expression Expression | Subtract Expression Expression | Multiply Expression Expression | Divide Expression Expression deriving Show eval :: Expression -> Float eval (Val x) = x eval (Add x y) = eval x + eval y eval (Multiply x y) = eval x * eval y eval (Subtract x y) = eval x - eval y eval (Divide x y) = eval x / eval y
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