[Haskell-cafe] SYB <<looping>> very, very mysteriously

[David Fox]

I have created an entry in the syb-with-class issue database
here:http://code.google.com/p/syb-with-class/issues/detail?id=3

I attached a version of the code with the necessary bits of
Happstack.Data.Default included in-line.

On Thu, Dec 3, 2009 at 2:50 PM, Jeremy Shaw <jeremy at n-heptane.com> wrote:
> I have the following program which loops under GHC 6.10.4:
>
> http://www.hpaste.org/fastcgi/hpaste.fcgi/view?id=13561#a13561
>
> {-# LANGUAGE DeriveDataTypeable, FlexibleInstances, MultiParamTypeClasses,
> UndecidableInstances #-}
> module Main where
>
> import qualified Data.Data as Data
> import Data.Typeable (Typeable)
> import Happstack.Data.Default
> import Data.Generics.SYB.WithClass.Basics
> import Data.Generics.SYB.WithClass.Instances ()
>
> data Proposition = Proposition Expression  deriving (Show, Data.Data,
> Typeable)
> data Expression = Conjunction (Maybe Expression) deriving (Show, Data.Data,
> Typeable)
>
> -- instance (Sat (ctx [Expression]), Sat (ctx Expression), Sat (ctx
> Proposition)) => Data ctx Proposition where
> instance Data DefaultD Proposition where
>    gunfold _ k z c =
>        case constrIndex c of
>          1 -> k (z Proposition)
> instance Default Proposition
>
> constrExpr :: Constr
> constrExpr = mkConstr dataTypeExpr "Conjuction" [] Prefix
>
> dataTypeExpr :: DataType
> dataTypeExpr = mkDataType "Expression" [constrExpr]
>
> instance ( Data ctx [Expression]
>         , Sat  (ctx Expression)
>         , Sat  (ctx (Maybe Expression))) => Data ctx Expression where
> {-
> instance Data DefaultD Expression where
> -}
>    gunfold _ k z c =
>        case constrIndex c of
>          1 -> k (z Conjunction)
>    dataTypeOf _ _ = dataTypeExpr
>
> instance Default Expression
>
> e :: Expression
> e =  defaultValueD dict
>
> main = print e
>
> I wish to explain the *many* ways in which it is mysterious. If you load the
> program into GHCi and evaluate 'e' it will hang. If you compile the program
> and run it, it will output <<loop>>. This behavior seems annoying, but not
> very weird. But, here is where it gets fun:
>
> 1. if you load the program into GHCi and eval 'e' it will hang. But, if you
> load the program and type, '(defaultValueD dict) :: Expression' at the
> prompt, it works fine!
>
> 2. if you remove the (Data DefaultD Proposition) instance, it  works fine.
> (Even though Expression does not refer to Proposition in any way)
>
> 3. if you simply change the definition of 'gunfold' in the 'Data ctx
> Proposition' instance to, error "foo". The application works fine. That's
> right, if you change the body of a function that isn't even being called,
> evaluating 'e' starts working. (Even though Expression does not refer to
> Proposition in any way. And even though that gunfold instance is never
> actually called).
>
> 4. if you change the constraint on, Data ctx Expression,  from (Data ctx
> [Expression]) to (Data ctx Expression) it works fine. (Or remove it all
> together).
>
> 5. if you change 'instance (Data DefaultD Proposition) where' to the line
> above it which is commented out, it works fine.
>
> 6. if you change the type of Proposition to, data Proposition = Proposition
> (Expression, Expression), then it works fine.
>
> So far I have only tested this in GHC 6.10.4.
>
> Any idea what is going on here? I can't imagine how changing the body of
> functions that aren't being called would fix things...
>
> - jeremy
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