Simple interpreter using Haskell

Question

I am still learning Haskell and the question seems very basic but it's been stomping me for a while.

Given the following

data Term =
    Number Integer
  | Abs Term
  | Plus Term Term
  | Mult Term Term
  deriving (Eq, Show)

I want to create an interpreter such that if I call, for example,

evaluate (Plus (Number 10) (Number 10))

it will return 20. Below is what I have so far

myabsolute :: Integer -> Integer
myabsolute n = if n >= 0 then n else -n

evaluate :: Term -> Integer
evaluate Plus = evaluate(t1) + evaluate(t2)
evaluate Mult = evaluate(t1) * evaluate(t2)
evaluate Abs = myabs(evaluate(t1))
evaluate _ = 0

I am confused how to get the Number Integer part working since I do not know how to extract the numbers and map them to t1 and t2 in Plus, Mult, and Abs.

Since I am still learning, if I am doing something completely wrong please let me know!

ErikR · Accepted Answer · 2014-11-09 06:12:39Z

7

Here's how the Plus case would work:

evaluate :: Term -> Integer
evaluate (Plus t1 t2) = evaluate t1 + evaluate t2
...

The Plus constructor has two terms, so we need two names in the pattern (t1 and t2). In this case both t1 and t2 will be values of type Term which is why the expressions evaluate t1 and evaluate t2 make sense.

The Number constructor, however, only has one parameter, e.g. Number 3, so its case will look like:

evaluate (Number x) = ...use x here...

Here the variable x will have type Integer.

answered Nov 9, 2014 at 6:12

ErikR

52.2k9 gold badges78 silver badges136 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

user3277633 Over a year ago

thanks for the help! Right now I got my plus, mult, and number working. For some reason my abs is giving mean " Equations for ‘evaluate’ have different numbers of arguments " error, do you know what the problem is? I've updated evaluate Abs t1 = myabs(evaluate(t1))

Collectives™ on Stack Overflow

Simple interpreter using Haskell

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related