Typeable and 'forall' in data constructors

[Simon Peyton-Jones]

PS: You can use deriving( Typeable )even when there's an existential.
It's just deriving( Data ) you have to do by hand.

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Akos Korosmezey
| Sent: 21 April 2004 13:23
| To: glasgow-haskell-users at haskell.org
| Subject: Typeable and 'forall' in data constructors
| 
| I am tying to write a Term class with function application:
| 
|    data (Typeable a) => Term a =
|        Const a |
|         LVar Int |
|         forall b. Typeable b => App (Term (b -> a)) (Term b) |
|          Lam (Term a)
| 
| Because 'forall' is present, ghc refuses to derive Typeable and Data
for
| Term. I tried to implement them:
| 
|    instance (Typeable a) => Typeable (Term a) where
|        typeOf w = mkAppTy (mkTyCon "Term.Term") [typeOf (undefined ::
a)]
| 
|    instance (Typeable a) => Data (Term a) where
|        toConstr (Const _) = mkConstr 1 "Const" Prefix
|        toConstr (LVar _) = mkConstr 3 "LVar" Prefix
|        toConstr (App _ _) = mkConstr 4 "App" Prefix
|        toConstr (Lam _) = mkConstr 5 "Lam" Prefix
| 
| But ghc 6.2.1 returns with error on the line 'toConstr (App _ _)...':
| parse error on input `b'. How can this be fixed?
| Thank you
| 
| Akos Korosmezey
| 
| 
| 
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