Question

I am a new learner in Haskell, the notion of Mutually recursive types confused me a lot.

Here is an example:

data BoolExpr
     = BoolConst Bool
     | BoolOp BoolOp BoolExpr BoolExpr
     | ConpOp CompOp IntExpr IntExpr

Why here are two BoolOp and CompOp?

I know that the first BoolOp is a data constructor, but how about the second BoolOp?

Answer 1

You’re right that the first BoolOp is a constructor; the second says that the constructor takes a value of type :: BoolOp. The full type is:

BoolOp :: BoolOp -> BoolExpr -> BoolExpr -> BoolExpr

So I’d expect there’s some code like this lying around:

data BoolOp = BoolAnd  |  BoolOr  |  BoolXor

So to use this constructor, you’d need to give it a BoolOp and two BoolExprs:

myBoolExpr :: BoolExpr
myBoolExpr = BoolOp  BoolOr  (BoolConst True)  (BoolConst False)