Question

Can anyone explain this error:

Syntax error in instance head (constructor expected)

class Nullable v where
  default_val :: v


instance Num a => Nullable a where  -- error reported here
  default_val = 0::a

Thanks

Answer 1

First off, hackage has you covered.

Secondly, don’t write instances of the form

instance OtherClass a => Class a where

they don’t work like you want. (YMMV with compilers other than GHC, but I can’t see that it’s a good idea even then). Presumably your intent is to create several instances, something like:

instance Num a => Nullable a where  -- error reported here
  default_val = 0::a

instance Bounded a => Nullable a where
  default_val = minBound

You can think of type class constraints as an extra argument to a function, like this:

instance Num a => Nullable a where
  default_val = NumDict a -> 0::a

instance Bounded a => Nullable a where
  default_val = BoundedDict a -> minBound

Then ghc actually sees these class instances like this:

instance Nullable a where
  default_val = NumDict a -> 0::a

instance Nullable a where
  default_val = BoundedDict a -> minBound

Now, which instance should the compiler choose? There are two instances that both claim to be valid for all types. So there’s a compiler error.

This is a problem even if you one have one type class based instance. Suppose that you have these instances:

instance Num a => Nullable a where
  default_val = 0::a

instance Nullable String where
  default_val = ""

instance Nullable BS.ByteString where
  default_val = BS.empty

The first is still considered valid for all types, so ghc says that it needs the OverlappingInstances extension to use them all. That’s not entirely evil. But when you try to make any use of this, before long ghc will require another extension, IncoherentInstances.

Lots of people are afraid to use UndecidableInstances, but that fear is misplaced. The worst that can happen with UndecidableInstances is that compiling won’t terminate (but it usually does). IncoherentInstances is the extension which should inspire fear, as it will bring doom upon your code. If GHC says that you have to enable IncoherentInstances, it means you need to change your code.

tl;dr

Don’t write instances of the form

instance OtherClass a => Class a where

They don’t do what you want.