[Haskell-cafe] Problem with type signature in local function
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 where addToN1 :: Num a = a - a addToN1 number = n1 + number ghc says Inferred type is less polymorphic than expected, and justly so, because the type variable a in addToN1 must of course be the same as in add. Is there a way to specify this? Regards, Chris ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problem with type signature in local function
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.)
{-# OPTIONS -fglasgow-exts #-}
add :: Num a = a - a - a
add n1 n2 = addToN1 n2
where addToN1 :: a - a
addToN1 number = n1 + number
... even though I would not claim that this is a really strong example
of lexically scoped variables, but you probably just simplied a real
code example.
And most of my recent code I use lexically scoped type variables
extensively. They are great.
(In principle, one does not need them at the cost of encoding. Using
asTypeOf and friends ...)
Merry Xmas,
Ralf
Christian Hofer wrote:
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
where addToN1 :: Num a = a - a
addToN1 number = n1 + number
ghc says Inferred type is less polymorphic than expected, and justly
so, because the type variable a in addToN1 must of course be the
same as in add. Is there a way to specify this?
Regards,
Chris
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Re: [Haskell-cafe] Problem with type signature in local function
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 #-}
add :: Num a = a - a - a
add (n1::a) n2 = addToN1 n2
where addToN1 :: a - a
addToN1 number = n1 + number
Cheers,
Arthur
On 24-dec-04, at 13:04, Ralf Laemmel wrote:
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.)
{-# OPTIONS -fglasgow-exts #-}
add :: Num a = a - a - a
add n1 n2 = addToN1 n2
where addToN1 :: a - a
addToN1 number = n1 + number
... even though I would not claim that this is a really strong example
of lexically scoped variables, but you probably just simplied a real
code example.
And most of my recent code I use lexically scoped type variables
extensively. They are great.
(In principle, one does not need them at the cost of encoding. Using
asTypeOf and friends ...)
Merry Xmas,
Ralf
Christian Hofer wrote:
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
where addToN1 :: Num a = a - a
addToN1 number = n1 + number
ghc says Inferred type is less polymorphic than expected, and
justly so, because the type variable a in addToN1 must of course be
the same as in add. Is there a way to specify this?
Regards,
Chris
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[Haskell-cafe] Re: Ignorant begginer question
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? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANN: MissingH 0.8.0 (Festive Lambda)
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 here. Not because I think my idea is better, but because: 1) It was most compatible with existing Haskell code, 2) I know how to implement it in a portable manner :-) If you are curious, there are haddock files at http://quux.org/devel/missingh/html/index.html -- John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problem with type signature in local function
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 (implementing my Java
homework in the imperative language of my choice)...
Merry christmas,
Chris
Am 24.12.2004 um 14:33 schrieb Arthur Baars:
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 #-}
add :: Num a = a - a - a
add (n1::a) n2 = addToN1 n2
where addToN1 :: a - a
addToN1 number = n1 + number
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Re: [Haskell-cafe] Re: Ignorant begginer question
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 constructor, written infix. Infix constructors all begin with ':', I think. Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Ignorant begginer question
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
Re: [Haskell-cafe] Re: Ignorant begginer question
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 = ... | Expr `In` [Expr] | Expr `And` Expr | Expr `Or` Expr ... case e of e1 `In` es - ... Best regards, Tomasz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
