Hello Cafe,
say we take these standard definitions:
> {-# LANGUAGE GADTs, TypeOperators, TypeFamilies, ScopedTypeVariables #-}
> data a :=: b where
> Refl :: a :=: a
> subst :: a :=: b -> f a -> f b
> subst Refl = id
Then this doesn't work (error message at the bottom):
> inj1 :: forall
iving' clause; the former
generates the code regardless of how weird the type is and lets the
typechecker decide. For your GADT, this succeeds).
Cheers,
Daniel Schüssler
On 2011-August-04 Thursday 08:57:46 Tim Cowlishaw wrote:
> Hi all,
>
> I've been writing a DSL to describe securit
On 2011-June-27 Monday 10:15:28 Yitzchak Gale wrote:
> The biggest shortcoming, in my opinion, is that the documentation
> assumes that the reader is very familiar with the Haskell type
> system, and with viewing type signatures and instance lists as an
> integral and central part of the documentat
ut.
>
>
>
> - Original Message -
>
> > From: Daniel Schüssler
> > To: haskell-cafe@haskell.org
> > Cc: Guy
> > Sent: Thursday, 9 June 2011, 2:06
> > Subject: Re: [Haskell-cafe] Type Constraints on Data Constructors
> >
Correction: I meant
data Baz f a = Baz (Foo f => f a)
(Dropped the 'forall', which would make the inner 'f' have nothing to do with
the type parameter 'f' of 'Baz')
On 2011-June-09 Thursday 01:07:13 Daniel Schüssler wrote:
> Hello,
>
> y
Hello,
you might be thinking of this type?
{-# LANGUAGE Rank2Types #-}
class Foo f where
foo :: a -> f a
data Baz f a = Baz (forall f. Foo f => f a)
instance Foo (Baz f) where
foo a = Baz (foo a)
Maybe the difference between Bar and Baz ist best explained by writing it with
an expl
Hello,
for reference, said instance is:
> instance Monad m => Monad (Iteratee a m) where
> return x = yield x (Chunks [])
>
> m0 >>= f = ($ m0) $ fix $
> \bind m -> Iteratee $ runIteratee m >>= \r1 ->
> case r1 of
> Co
Hello,
assuming you mean avoiding the import of Data.Map in the module *using* x, you
can use name quotations:
A.hs:
> {-# LANGUAGE TemplateHaskell #-}
>
> module A where
>
> import Data.Map
> import Language.Haskell.TH
>
> x = varE 'empty
>
B.hs:
> {-# LANGUAGE TemplateHaskell #-}
> m
Hello,
turns out that you can define the group operation of the symmetric group on 3
elements in this abstract way (via the isomorphism to the group of bijective
functions from a three-element type to itself):
s3mult g2 g1 = fromFun (toFun g2 . toFun g1)
and convince GHC to compile it
Hi,
On 2011-February-27 Sunday 16:20:06 Edward Amsden wrote:
> Secondly,
>
> I'd like to get to a GHC session that just has, say, Prelude in scope
> so I can use dynCompileExpr with "show" etc, but I cannot figure out
> how to bring it into scope. The closest I got was to get GHC
> complaining th
On Thursday 18 February 2010 11:26:02 Ozgur Akgun wrote:
> I've no idea about the GLPK system.
>
> But, isn't it the case that you can transform any linear inequality into a
> linear equality and a slack (or excess) variable?
Well yes, but the slack variables are constrained to be nonnegative, wh
Hello Alberto,
Thank you! I don't have a problem calling for LP at hand right now, but some
time ago I was looking for such a package. Now I know where to look next time
:)
Greetings,
Daniel
On Wednesday 24 February 2010 11:07:08 Alberto Ruiz wrote:
> I have uploaded to hackage an interface
On Sunday 14 February 2010 17:02:36 Henk-Jan van Tuyl wrote:
> The symbols that are not specified in a library can be found here:
>http://www.haskell.org/haskellwiki/Keywords
> Hoogle used to show links to this page, when a keyword was searched, but
> not anymore.
>
This isn't Haskell 98 only
Hi all,
On Wednesday 25 November 2009 22:46:42 Luke Palmer wrote:
> I feel like this should be qualified. Type classes are not for name
> punning ; you wouldn't use a type class for the method bark on types
> Tree and Dog. But if you have a well-defined *structure* that many
> types follow, then
On Sunday 15 November 2009 13:05:08 Nicolas Pouillard wrote:
> Excerpts from Daniel Schüssler's message of Sun Nov 15 07:51:35 +0100 2009:
> > Hi,
>
> Hi,
Hi,
>
> > -- Invariant 1: There are never two adjacent Lefts or two adjacent Rights
>
> [...]
>
> > normalize (Left a0 : Left a1 : as) = L
Hi,
> - Product (a,b) and co-product (Either) of monoids
the coproduct of monoids is actually a bit tricky. It could be implemented
like this:
-- |
-- Invariant 1: There are never two adjacent Lefts or two adjacent Rights
-- Invariant 2: No elements (Left mempty) or (Right mempty) allowed
new
Hi,
On Friday 13 November 2009 21:08:42 Neil Mitchell wrote:
> In HLint I have a bracketing module, which has served me well. Please
> take any ideas you need from it -
> http://community.haskell.org/~ndm/darcs/hlint/src/HSE/Bracket.hs . In
> particular, given a fully bracketed expression, I can c
Hi Deniz,
> Cool stuff, I will probably be using this!
thanks :)
>
> In my opinion, the naming convention is a bit inconsistent. Extension
> snippets all begin with "-x" but imports begin with "imp". I'd prefer
> seeing import snippets begin with "-i" and use names easier to
> remember, e.g. ins
Hi List,
this is rather trivial, but maybe someone else finds these useful:
darcs get http://code.haskell.org/~daniels/haskell-snippets/
Especially the LANGUAGE ones have saved me quite some typing :) Additions
welcome.
Usage: If not already installed, get YASnippet:
http://code.google.com/p/
Hi,
On Thursday 22 October 2009 09:47:32 Martijn van Steenbergen wrote:
> Bonjour café,
>
> > data ExprF r
> > = Add r r
> >
> > | Sub r r
> > | Mul r r
> > | Div r r
> > | Num Int
>
> This is a well-known pattern that for example allows nice notation of
> morphisms. But
On Tuesday 13 October 2009 02:46:29 Erik de Castro Lopo wrote:
> Hi all,
>
> I've just received the following error message:
>
> headers.hs:6:7:
> Could not find module `Control.Monad.Identity':
> it was found in multiple packages: transformers-0.1.4.0 mtl-1.1.0.2
>
> I'm trying to u
Hi Sjoerd,
I don't know the cause of the problem, but if I add this rule, it works:
{-# RULES
"inline_map" forall g x. map g x = transform (. g) x
-#}
maybe, for whatever reason, the 'map' is inlined "too late" for the
transform/transform rule to see it?
Greetings,
Daniel
On Monday 22 Ju
Hi,
ParsecT with m=IO? Your 'do' block would become:
do
i <- getInput
included <- liftIO readI -- import Control.Monad.Trans for liftIO
setInput included
a <- my_str
setInput i
b <- my_str
return $ a ++ " //\n\n " ++ b
where
readI
Hi,
meh, I just realised that there is no sensible way to actually
introduce/eliminate the generated types. I'm attaching a revised version with
fromList/toList functions. Maybe the vector type should be polymorphic and be
an instance of Functor, Monad and Foldable? But then we really depend on
Hello!
On Monday 18 May 2009 14:37:51 Kenneth Hoste wrote:
> I'm mostly interested in the range 10D to 100D
is the dimension known at compile-time? Then you could consider Template
Haskell. I wrote up some code for generating the vector types and vector
subtraction/inner product below, HTH. One
Hello!
The problem is that it's impossible to infer the SomeClass instance from the
type SomeRole. If you do "print role", which instance should it use?
I can think of two ways around it:
-- 1. (dummy parameter)
--
data SomeRole a = Role1 | Role2 | Role3 deriving Sho
On Tuesday 17 March 2009 21:03:21 Rick R wrote:
> QED
Hmm? Maybe if confusingness was to be demonstrated, but not bugginess. Both
possibilities will result in the same total preordering (defined by (x
`betterThanOrEq` y) iff (numberInCombobox x <= numberInCombobox y)), and
(AFAIK) only this ord
(correction of the example)
(105: ) (106: A) (107: X,B) (108: C,D) (109: E ) (110: )
moving down X will result in either
(105: A) (106: B) (107: X ) (108: C,D) (109: E ) (110: )
or equivalently
(105: ) (106: A) (107: B ) (108: X ) (109: C,D) (110: E)
__
Hi,
> Even worse, the buttons for moving items up and down are buggy - at
> least on my browser (Firefox 3.1 beta 2 on Linux). They sometimes
> reorder my other votes! Even assuming that the list box code is not
> buggy (which I now doubt), not being able to use the buttons makes this
> form almos
> Although maybeRead was proposed, I cannot find it:
here's a replacement...
http://hackage.haskell.org/packages/archive/safe/0.2/doc/html/Safe.html#v%3AreadMay
Greetings,
Daniel
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Haskell-Cafe@haskell.org
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Hi,
On Sunday 01 March 2009 14:26:42 Nicu Ionita wrote:
> [...]
> movesFromWord8s (f:t:ws) = (f, t) : movesFromWord8s ws
> moverFromWord8s _ = []
> [...]
> Are there possible solutions or workarounds?
for the particular problem of having to repeat the function name, you could
use "case":
movesF
, or anything else I tried with "#" on the end of
> names, to compile with ghci. In the example below, I get a parse error
> at the line defining d2f. Is this a real code snippet?
>
>
> Cheers,
> Dylan
>
> On Feb 10, 2009, at 2:46 AM, Daniel Schüssler wrote:
> &g
Hi,
On Wednesday 28 January 2009 04:30:07 John Goerzen wrote:
> On Tue, Jan 27, 2009 at 09:41:30PM -0500, wren ng thornton wrote:
> > I once again point out that realToFrac is *wrong* for converting from
> > Float or Double.
> >
> > > realToFrac (1/0::Float) ::Double
> >
> > 3.40282366920
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