mutually recursive types in haskell

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?

Where is this code from? It'll be easier to explain with more context. — amindfv
– amindfv, Commented Aug 21, 2012 at 1:01
I don't see mutually recursive types; I just see a single recursive data type, BoolExpr. Am I missing something? — Matt Fenwick
– Matt Fenwick, Commented Aug 21, 2012 at 1:09

Matt Fenwick · Accepted Answer · 2012-08-21 01:04:34Z

7

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)

answered Aug 21, 2012 at 1:04

Matt Fenwick

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3 Comments

@Kevin quick, say it fives times fast while holding your tongue!

@MattFenwick so i can change the name of BoolOp, can i ? like: data BoolExpr = a BoolOp BoolExpr BoolExpr. is that correct?

@user1611996 you can change it, but constructors have to start with a capital letter, so a won't work.