CODEWITHSUNDEEP
haskellinterpreterabstract-syntax-tree
user3277633
user3277633

Reputation: 1923

Simple interpreter using Haskell

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!

Upvotes: 2

Views: 115

Answers (1)

ErikR
ErikR

Reputation: 52057

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.

Upvotes: 7

Related Questions