Re: monomorphism etc.

[Fergus Henderson]

On 12-Nov-1998, Lennart Augustsson [EMAIL PROTECTED] wrote:
 
 [...] if you dislike tuples you can use nested pairs

At the cost of losing a little type-safety.

-- 
Fergus Henderson [EMAIL PROTECTED]  |  "Binaries may die
WWW: http://www.cs.mu.oz.au/~fjh  |   but source code lives forever"
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Re: monomorphism etc.

[Lennart Augustsson]

 I think it would be
 really nice if it were possible to create a container capable of
 containing any number of objects of any number of types  It would
That's not possible in Haskell.  Since you want an any number
of different types in this new type it would need to have a variable
number of type variables, which is impossible.

But if you dislike tuples you can use nested pairs, like

  infixr 0 :.
  data T a b = a :. b

Now you can write

  'a' :. True :. "Hello" :. ()

which has type

  T Char (T Bool (T String ()))

But I guess that's not what you were after?

-- Lennart

Re: monomorphism etc.

[Lennart Augustsson]

  
  [...] if you dislike tuples you can use nested pairs
 
 At the cost of losing a little type-safety.
That's really a very minimal loss.  (And it's not really a loss of
type safety, just the possibilty of confusing a part of a "tuple" with
another "tuple").  I'd be more worried about the performance loss due
to the extra lifting for each nested pair.

But tuples should be used very sparingly anyway, in my opinion.
Define new types instead!

   -- Lennart

nested pairs (was Re: monomorphism etc.)

[Claus Reinke]

But if you dislike tuples you can use nested pairs, like

  infixr 0 :.
  data T a b = a :. b

Now you can write

  'a' :. True :. "Hello" :. ()

which has type

  T Char (T Bool (T String ()))

But I guess that's not what you were after?

-- Lennart

Fergus replied:
  [...] if you dislike tuples you can use nested pairs

 At the cost of losing a little type-safety.

Well, since no one else mentions it: It's fun to (try to) define some
functions over these ``heterogeneous lists''.

In which languages can you define, e.g., a length function?-)

Claus

PS. I've got a length function for ``heterogeneous lists'', as
they appear in nested pairs, in Hugs. However, it uses the
type system extensions available in 1.3c or in 1.4 [98 BETA].
How much can you do in plain Haskell??? in Cayenne?-)

Re: nested pairs (was Re: monomorphism etc.)

[Claus Reinke]

PS. I've got a length function for ``heterogeneous lists'', as
they appear in nested pairs, in Hugs. However, it uses the
type system extensions available in 1.3c or in 1.4 [98 BETA].
How much can you do in plain Haskell??? in Cayenne?-)

oops. 
On second thought, this length program uses only plain old Haskell?

[from a Hugs1.4June1998 session transcript]

  Main  l (True :. 'c' :. "length" :. ())
  3
  Main l ()
  0
  Main l (() :. ())
  1


What else have I missed?-)
Claus

monomorphism etc.

[David Feuer]

I tried and failed to understand the reasons given in the Report (1.4)
for the monomorphism restriction and other type things.  Could someone
explain what these mean and why they are needed? I think it would be
really nice if it were possible to create a container capable of
containing any number of objects of any number of types  It would
avoid the nastiness of a special definition for each tuple type and and
lead to more flexibility.  I suspect it is impossible, but don't know
why.


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