CODEWITHSUNDEEP
haskell
Chetan Yewale
Chetan Yewale

Reputation: 312

Getting Error "Not a Data constructor" when defining a new datatype in Haskell

I am trying to define a new datatype for directions. But, I am getting an error "Not a data constructor: left" when I load the file in ghci.

Here is the code snippet:

data Dir =   left
           | right 
           | up
           | down 

instance Show Dir where
 show left = "left"
 show right = "right"
 show up = "up"
 show down = "down"


-- initialpos is assumed to be valid one
move::Pos->Dir->Pos
move prevPos left = ( (-1)  $ fst prevPos  ,  snd prevPos)   
move prevPos right = ( (+1) $ fst prevPos ,  snd prevPos )
move prevPos up = ( fst prevPos, (+1) $ snd prevPos  )
move prevPos down = ( fst prevPos, (-1) $ snd prevPos )

Please help me in figuring out what is wrong.

Upvotes: 0

Views: 2928

Answers (2)

shree.pat18
shree.pat18

Reputation: 21757

As @chepner rightly points out, the data constructors should begin with capital letters. To address the next problem you run into i.e. Left and Right are ambiguous between your defined ones and those from Either, you can do this:

  1. Explicitly import Prelude and hide Either type constructor to avoid the ambiguity. This will let you continue using all the other 'default' functions from Prelude without issue

    import Prelude hiding (Either(..)) --Everything except Either which causes ambiguity issues

  2. (Optional) Do a qualified import of Data.Either if you wish to use it elsewhere in your code.

    import qualified Data.Either as E --You will be able to use Either by writing E.Either in your code

That said, you could avoid this trouble by renaming your data constructors if that is an option.

Upvotes: 2

chepner
chepner

Reputation: 532528

Ignoring infix data constructors, a data constructor must begin with a capital letter.

data Dir = Left | Right | Up | Down

Upvotes: 6

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