Hi,
in order to understand the reason for strange compiler error messages,
I often find it convenient to add type signatures to every function.
But sometimes this seems to be impossible when using type variables.
I try to give a very easy example:
add :: Num a = a - a - a
add n1 n2 = addToN1 n2
Timely question. :-)
GHC supports lexically scoped type variables.
And this support has just been _extended_ to be readily useful for you
(see discussion on the ghc list just a few days back).
With GHC CVS HEAD the following program works fine.
(Note that I removed the inner Num constraint.)
{-#
You could also do it with the current implementation of scoped type
variables. So you don't need to compile the CVS HEAD version. Note that
you need the pattern (n1::a) and/or (n2::a) to introduce the scoped
type variable, which was not necessary in Ralf's solution.
{-# OPTIONS -fglasgow-exts
On 2004-12-23, Stefan Holdermans [EMAIL PROTECTED] wrote:
Your problem right now is that the type Complex takes (needs) a type
argument. Its definitions is (module strictness flags):
data Complex a = a :+ a
What does the :+ mean here?
___
On 2004-12-24, John Goerzen [EMAIL PROTECTED] wrote:
* Virtualized I/O system
Use familiar functions to work on not just Handles but all sorts of
other types, including in-memory buffers. (HVIO module)
I should comment on this... I went with basically what I originally
asked about
Thank you all for your answers!
I indeed do not want to use CVS HEAD, but it's nice to know that there
will be a better solution some time. Until then I will write s.th. like
loop (residual :: MutableGraph state node weight) flow absValue = ...
which is completely sufficient for my purposes
On 24 Dec 2004, at 14:53, John Goerzen wrote:
On 2004-12-23, Stefan Holdermans [EMAIL PROTECTED] wrote:
Your problem right now is that the type Complex takes (needs) a type
argument. Its definitions is (module strictness flags):
data Complex a = a :+ a
What does the :+ mean here?
It's a data
Jules,
It's a data constructor, written infix. Infix constructors all begin
with ':', I think.
That's correct.
Regards,
Stefan
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
On Fri, Dec 24, 2004 at 03:14:34PM +, Jules Bean wrote:
It's a data constructor, written infix. Infix constructors all begin
with ':', I think.
Yes, but you can also use prefix, alphanumeric constructors as infix
by placing them in backticks. This can be nice sometimes:
data Expr =
