Haskell record syntax error: not in scope

Question

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?

Note that the record accessors are functions in the module scope. So key and value are now functions that might override any other definition. This is why you usually see treeValue instead of just value. — Sean Perry
– Sean Perry, Commented Apr 11, 2014 at 20:21

bheklilr · Accepted Answer · 2014-04-11 20:10:02Z

4

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.

edited Apr 11, 2014 at 20:10

answered Apr 11, 2014 at 19:59

bheklilr

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1 Comment

Boyd Stephen Smith Jr. Over a year ago

Nit-pick all constructors don't have to have the same fields. When not all fields are present on all constructors, you get accessors that are partial: Prelude> data Foo = Empty | One { only :: String } | More { than_ever :: [Int] } Prelude> :t Empty Empty :: Foo Prelude> :t One "Another" One "Another" :: Foo Prelude> :t More [2, 4] More [2, 4] :: Foo Prelude> only $ More [2, 4] "*** Exception: No match in record selector only Prelude> than_ever Empty *** Exception: No match in record selector than_ever

chi · Accepted Answer · 2014-04-11 23:20:37Z

1

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)

answered Apr 11, 2014 at 23:20

chi

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