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{-# LANGUAGE GADTs, KindSignatures #-}
-- See Trac #301
-- This particular one doesn't use GADTs per se,
-- but it does use dictionaries in constructors
module Expr1 where
data Expr :: * -> * where -- Not a GADT at all
Const :: Show a => a -> Expr a
-- Note the Show constraint here
Var :: Var a -> Expr a
newtype Var a = V String
instance Show (Var a) where show (V s) = s
--------------------------
e1 :: Expr Int
e1 = Const 42
e2 :: Expr Bool
e2 = Const True
e3 :: Expr Integer
e3 = Var (V "mersenne100")
--------------------------
eval :: Expr a -> a
eval (Const c) = c
eval (Var v) = error ("free variable `" ++ shows v "'")
{-
Up to here, everything works nicely:
\begin{verbatim}
*Expr0> eval e1
42
*Expr0> eval e2
True
*Expr1> eval e3
*** Exception: free variable `mersenne100'
\end{verbatim}
But let us now try to define a |shows| function.
In the following, without the type signature we get:
\begin{verbatim}
*Expr1> :t showsExpr
showsExpr :: forall a. (Show a) => Expr a -> String -> String
*Expr1> showsExpr e1 ""
"42"
*Expr1> showsExpr e2 ""
"True"
*Expr1> showsExpr e3 ""
"mersenne100"
\end{verbatim}
However, in the last case, the instance |Show Integer| was not used,
so should not have been required.
Therefore I would expect it to work as it is now, i.e.,
with the type signature:
-}
showsExpr :: Expr a -> ShowS
showsExpr (Const c) = shows c
showsExpr (Var v) = shows v
{-
We used to get a complaint about the |Const| alternative (then line
63) that documents that the constraint in the type of |Const| must
have been ignored:
No instance for (Show a)
arising from use of `shows' at Expr1.lhs:63:22-26
Probable fix: add (Show a) to the type signature(s) for `showsExpr'
In the definition of `showsExpr': showsExpr (Const c) = shows c
-}
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