CODEWITHSUNDEEP
haskellsyntaxfunction-callcustom-data-type
Carlos Romero
Carlos Romero

Reputation: 181

How to extract information from custom data type

I am new to Haskell and functional programming in general. I am currently learning about custom data types and have the following:

data Point = Pt Double Double deriving (Show, Eq)

getValue :: Point -> Double

getValue (Pt a _) = a

I am attempting to return only one of the double values from the custom data type but am running into a problem. My console gives me the following error whenever I try to extract one of my point values: "Couldn't match expected type Integer -> Integer -> t". Any ideas as to what I am doing wrong?

(edit from OP's comments:) I typed the following in GHCi 

test Pt 1.0 1.0

Here is the entire error message : 

• Couldn't match expected type ‘Integer -> Integer -> t’ 
      with actual type ‘Double’ 
• The function ‘test’ is applied to three arguments, 
      but its type ‘Point -> Double’ has only one 
  In the expression: 
      test Pt 1 1 
  In an equation for ‘it’: 
      it = test Pt 1 1 
• Relevant bindings include 
      it :: t (bound at <interactive>:497:1)

test is what I renamed getValue as.

Upvotes: 2

Views: 514

Answers (1)

Will Ness
Will Ness

Reputation: 71119

you clearly want to do test (Pt 1.0 1.0).

test Pt 1.0 1.0 with no parentheses is interpreted as applying the test function to the 3 arguments Pt, 1.0 and 1.0, which makes no sense and is why GHC is complaining. – Robin Zigmond, yesterday

Upvotes: 1

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