GADT records revisited

[Simon Peyton Jones]

It's a choice.  Consider

data Bar a where
  B1 :: { x :: b } -> Bar [b]
  B2 :: { x :: b } -> Bar [b]
  B3 :: Bar a

Now we can define a perfectly good selector
  x :: Bar [b] -> b
  x (B1 v) = v
  x (B2 v) = v

But this wouldn't work if the result types were different

data BadBar a where
  B1 :: { x :: b } -> Bar [b]
  B2 :: { x :: b } -> Bar b
  B3 :: Bar a


Now it's true that in your example the field mentions only *polymorphic* components, so there is a perfectly well-defined selector with the type you give.  Indeed, it could be a bit more complicated:

data FooBar x a where
  Foo :: { fooBar :: a } -> FooBar Char [a]
  Bar :: { fooBar :: a } -> FooBar Bool [a]

Then there is a reasonable selector with type

  fooBar :: FooBar b [a] -> a

So what is the general rule? When exactly is there a well-defined selector type, and what is that type?  Notice that in the type of fooBar we had to generalise over the Char/Bool difference, but maintain the [a] part.  Indeed it might all be part of one type:

data FooBar2 x where
  Foo2 :: { fooBar2 :: a } -> FooBar2 (Char, [a])
  Bar2 :: { fooBar2 :: a } -> FooBar2 (Bool, [a])

So now

  fooBar2 :: FooBar2 (x, [a]) -> a

where we generalise part of the type. 

So, on reflection, there must be a more permissive rule than the one GHC currently implements.  If someone wants to figure out the general rule, express it formally, say what the user manual would say, we could discuss whether the cost benefit ratio is good enough to be worth implementing.  

This seems interesting enough to make into a ticket.
https://ghc.haskell.org/trac/ghc/ticket/8673

Simon



| -----Original Message-----
| From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of "Philip K.F. Hölzenspies"
| Sent: 17 January 2014 06:51
| To: glasgow-haskell-users at haskell.org
| Subject: GADT records revisited
| 
| Dear all,
| 
| I was playing around with GADT-records again and ran into behaviour that
| I'm not sure is intentional. Given this program:
| 
| 
| 
| {-#LANGUAGE GADTs #-}
| 
| data FooBar x a where
|    Foo :: { fooBar :: a } -> FooBar Char a
|    Bar :: { fooBar :: a } -> FooBar Bool a
| 
| 
| GHC tells me this:
| 
| 
| foo.hs:3:1:
|      Constructors Foo and Bar have a common field `fooBar',
|        but have different result types
|      In the data declaration for `FooBar'
| Failed, modules loaded: none.
| 
| 
| The user guide does say (section 7.4.7): "However, for GADTs there is
| the following additional constraint: every constructor that has a field
| f must have the same result type (modulo alpha conversion)." So this
| behaviour is documented in the user guide. However, it seems reasonable
| that in the case above, where all the relevant variables are exposed in
| the result type of both constructors, this should be perfectly typeable.
| In other words, shouldn't GHC be able to derive a type that is simply:
| 
| fooBar :: FooBar x a -> a
| 
| ?
| 
| Is this something that was simply never implemented, but could be, or is
| this not decidable or prohibitively computationally complex?
| 
| Regards,
| Philip
| 
| 
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