[Haskell-cafe] Increasing Haskell modularity

[Gesh]

On 10/2/2014 5:21 PM, oleg at okmij.org wrote:
 > Given
 >> f2 :: Show a => a -> String
 >> f2 x =
 >>    let instance Show Int where show _ = "one"
 >>    in show x
 >
 > what should (f2 (1::Int)) return? One answer is that it should still
 > return "one" -- that is, local instances should sort of form a
 > closure (like local variables in the real closure). That is a good
 > answer, but may lead to unpleasant surprises because the fact of the
 > closure is not at all reflected in the type of the function.
 >
 > Consider
 >> data T = T
 >> f3 :: Show a => a -> String
 >> f3 x =
 >>    let instance Show T where show _ = "T"
 >>    in show x
 >
 > and take the closure semantics. The type says that to make (f3 T)
 > well-typed, there has to be an instance Show T in scope. The user of
 > f3 has no way to know of the private local instance that is closed
 > over (short of examining the source code). Since the user knows
 > there is no global instance T, the user attempts to define in. The
 > user will be quite surprised to discover that (f3 T) is well-typed
 > but their global instance Show T has no effect.
That is true, but even today one can write overly-constrained
expressions that don't actually need these constraints. How is `f3`
above any different from
 > foo :: Show a => a -> String
 > foo = const "Fooled you, I didn't need Show after all."

However, one could write a more precise type for `f3` which reflects
the fact that a `Show` instance is unnecessary for `T`, as follows
 > class Foo a where
 > instance Foo T where
 > instance Show a => Foo a where
 > f3 :: Foo a => a -> String

(Of course, this requires OverlappingInstances)
In fact, it would be useful to have syntactic sugar for this sort of
constraint disjunction, for cases like this. We might even extend
GHC's type checker to infer such disjunctions, so that `f3`'s type
would be inferred to be `Show a \/ (a ~ T) => a -> String`.

Thanks for the in-depth analysis and further reading,

Gesh

P.S. You might want to change your email client's settings, as you
      don't seem to be sending In-Reply-To headers, which makes
      threading horrible.