Explanation of class instance declarations-CodePudding

I am following a tutorial and found this code:

data A = B | C deriving(Eq)

class K a where
 f :: a -> Bool

instance K A where
 f x = x == C
 f _ = False
 
call = f B

Why do I need f _ = False? I get the same result without it.

CodePudding user response：

The answer is simply: you don't need f _ = False here. In fact, if you compile with -Wall then the compiler will warn you that this clause is redundant, because the f x = ... clause already catches everything.

If the tutorial told you to have that extra clause, well, it's wrong.

CodePudding user response：

As pointed out, it's not necessary.

You might need (or want) that line, though, if you had a slightly different definition, one that does not require an Eq instance:

data A = B | C

class K a where
 f :: a -> Bool

instance K A where
 f C = True
 f _ = False

Instead of comparing x to C, you can match the argument directly against C, then define f to return False for all other values. This makes more sense if there were more constructors that could produce False.

data A' = B | C | D

instance K A' where
    f C = True
    f _ = False  -- in place of f B = False and f D = False