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haskelltypesrecord
eswarp25
eswarp25

Reputation: 155

Haskell record syntax error: not in scope

I am trying to define a multi-way tree data type in ghci. Each node has a key and a value. Here is the related code:

    data Tree k v = Empty |
                    N {key::k,
                       value::v} [Tree k v]
                  deriving (Show, Eq)

Here is corresponding output from Ghci:

    Not in scope: `key'
    Not in scope: `value'

I checked the syntax of the record by defining the tree node record separately using the following code:

   data Node k v = Node { key::k
                        , value::v}
                 deriving (Show, Eq)

This works as expected. What is causing my Tree data type definition to throw this error? Am I missing something related to record syntax when used to define an alternate value constructor?

Upvotes: 1

Views: 976

Answers (2)

chi
chi

Reputation: 116139

If you want to use a separate node type, you may be looking for something like

data Tree k v = Empty | N (Node k v) [Tree k v]
              deriving (Show, Eq)

data Node k v = Node { key::k
                     , value::v}
              deriving (Show, Eq)

Upvotes: 1

bheklilr
bheklilr

Reputation: 54058

When using the record syntax with ADTs, all constructors must have the same fields. Additionally, you're attempting to combine record and normal constructor syntax with Node { key :: k, value :: v } [Tree k v]. Instead, you'll have to resort to normal positional arguments to your constructor like

data Tree k v
    = Emtpy
    | N k v [Tree k v]
    deriving (Eq, Show)

You could use record syntax with ADTs as

data Tree k v
    = RedNode   { key :: k, val :: v, children :: [Tree k v] }
    | BlackNode { key :: k, val :: v, children :: [Tree k v] }
    deriving (Eq, Show)

But I would recommend making a separate data type as

data Color = Red | Black deriving (Eq, Show)

data Tree k v = Node
    { color :: Color
    , key :: k
    , val :: v
    , children :: [Tree k v]
    } deriving (Eq, Show)

Because then if you extend Color, you have less typing, and I would argue that it's more idiomatic.

Upvotes: 4

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