RE: Haskell-98 Quiz

1999-04-26 Thread Magnus Carlsson 

Mark P Jones writes:
  | 2. Is there a way to modify the signatures to make it legal?
  
  Not that I can see!
  
  Personally, I think you've found a bug in the Haskell report!  But, as
  it stands, others can reasonably say this is a bug in Hugs 98 ... I guess
  we should modify the typechecker to reject this kind of program, at least
  when Hugs is running in Haskell 98 mode.  But it seems a shame to do all
  that work for a check that people might prefer to do without :-(

I think I prefer the Hugs 98 behaviour :-)

/M

RE: Haskell-98 Quiz

1999-04-25 Thread Mark P Jones 

| Here are some questions for the Haskell-98 enthusiasts.

Are implementors allowed to answer too? :-)  It was a nice little puzzle!
 
| 1. Why is the following declaration group illegal?
| 
|   f :: String
|   f = g 1 ++ g True
| 
|   g :: Show a = a - String
|   g x = fst (show x, show f)

Well according to my copy of the Haskell report, Section 4.5.2 on p56:

 "If the programmer supplies explicit type signatures for more than one
  variable in a declaration group, the contexts of these signatures must
  be identical up to renaming of the type variables."

| 2. Is there a way to modify the signatures to make it legal?

Not that I can see!

Personally, I think you've found a bug in the Haskell report!  But, as
it stands, others can reasonably say this is a bug in Hugs 98 ... I guess
we should modify the typechecker to reject this kind of program, at least
when Hugs is running in Haskell 98 mode.  But it seems a shame to do all
that work for a check that people might prefer to do without :-(

All the best,
Mark

Re: Haskell-98 Quiz

1999-04-23 Thread Christian Sievers 

Magnus Carlsson wrote:

 Here are some questions for the Haskell-98 enthusiasts.

I'm not sure if I'm a Haskell-98 enthusiast, I still call myself 
a Haskell enthusiast.

 1. Why is the following declaration group illegal?
 
   f :: String
   f = g 1 ++ g True
 
   g :: Show a = a - String
   g x = fst (show x, show f)

I don't see why it should be illegal, but then nor does Hugs 98.
It is happy with this definition and gives "1True" for f.
So if you found a subtle strange thing in Haskell 98, you also found a 
bug in Hugs.


Christian Sievers

Haskell-98 Quiz

1999-04-23 Thread Magnus Carlsson 

Here are some questions for the Haskell-98 enthusiasts.

1. Why is the following declaration group illegal?

  f :: String
  f = g 1 ++ g True

  g :: Show a = a - String
  g x = fst (show x, show f)

2. Is there a way to modify the signatures to make it legal?

/M