CODEWITHSUNDEEP
haskellghci
Alex
Alex

Reputation: 287

How to call a function with specific data structure in ghci

I have this function which calculates the add

data Expr
  = Num Double
  | Add Expr Expr
  | Sub Expr Expr
  deriving (Show)



  eval :: Expr -> Maybe Double
    eval (Num a) = if a < 0 then 
    eval (Add a b) = case eval a 
      Nothing -> Nothing
      Just a1 -> case eval b of
        Nothing -> Nothing
        Just b1 -> Just (a1 + b1)

while calling it like this, it returns error:

eval Add (Expr(1), Expr(2))

:30:20: error: Data constructor not in scope: Expr :: t1 -> b0

Upvotes: 1

Views: 229

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476557

Your Add (Expr(1), Expr(2)) is not an expression for an Expr. First of all you need Num to construct an Expr, not an Expr since that is a type constructor, not a data constructor:

Add (Num 1) (Num 2)

Furthermore there should be no comma between the two, and we need brackets to pass the entire Expr we need parenthesis around the entire Add … block, so:

eval (Add (Num 1) (Num 2))

In your eval function it looks like something is missing after the if a < 0 then …

You can also implement eval (Add a b) as:

eval :: Expr -> Maybe Double
eval (Num a) = if a < 0 then … else …
eval (Add a b) = (+) <$> eval a <*> eval b

Upvotes: 6

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