Record syntax is illegal. Binary Tree in haskell

Question

I defined the tree type with record syntax way.

data Tree a = Null | Node a {lTree:: Tree a, rTree :: Tree a}

However, when I load it, it shows an error:

• Record syntax is illegal here: {lTree :: Tree a, rTree :: Tree a}
    • In the type ‘{lTree :: Tree a, rTree :: Tree a}’
      In the definition of data constructor ‘Node’
      In the data declaration for ‘Tree’
  |
2 | data Tree a = Null | Node a {lTree:: Tree a, rTree :: Tree a}
  |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

By the way, it's inherently dangerous to use record syntax with multiple constructors. It makes the record accessors into partial functions. — Carl
– Carl, Commented Oct 29, 2018 at 15:07

willeM_ Van Onsem · Accepted Answer · 2018-10-29 09:47:45Z

10

The a should also be part of the record syntax, for example:

data Tree a = Null | Node {valTree :: a, lTree:: Tree a, rTree :: Tree a}

You can not "mix" record syntax with positional syntax as is specified by the Haskell grammar in the Haskell '98 report:

constr  -> con [!] atype1 ... [!] atypek  (arity con = k, k>=0)
        |  (btype | ! atype) conop (btype | ! atype)  (infix conop)
        |  con { fielddecl1 , ... , fielddecln }  (n>=0)

answered Oct 29, 2018 at 9:47

willeM_ Van Onsem

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