Re: [Haskell-cafe] Comparing functions
Thanks Roman. Tried it and implemented, but had troubles until I realized that
for String, 10 test take quite long. :-)
However, I decided to solve this problem in a more natural way
Original Message
Subject: Re: [Haskell-cafe] Comparing functions
From: Roman Cheplyaka r...@ro-che.info
To: Vlatko Basic vlatko.ba...@gmail.com
Cc: Haskell-Cafe haskell-cafe@haskell.org
Date: 11.07.2013 20:10
* Vlatko Basic vlatko.ba...@gmail.com [2013-07-11 19:33:38+0200]
Hello Cafe,
I have
data CmpFunction a = CF (a - a - Bool)
that contains comparing functions, like ==, , ..., and I'm trying
to declare the Show instance for it like this
instance Show (CmpFunction a) where
show (CF (==)) = ==-- no good
show f = case f of-- no good also
CBF (==) - ==
_ - Other
but compiler complains for both with
This binding for `==' shadows the existing binding
imported from `Prelude' at src/Main.hs:6:8-11
(and originally defined in `ghc-prim:GHC.Classes')
Is it possible at all to compare two functions or how to solve this
problem, to show some string for a specific function?
Depending on why you need that...
{-# LANGUAGE FlexibleContexts, UndecidableInstances, FlexibleInstances #-}
import Test.SmallCheck
import Test.SmallCheck.Series
import Test.SmallCheck.Drivers
import Control.Monad.Identity
import Data.Maybe
data CmpFunction a = CF (a - a - Bool)
feq :: (Show a, Serial Identity a) = CmpFunction a - CmpFunction a - Bool
feq (CF f1) (CF f2) =
isNothing $
runIdentity $
smallCheckM 10 (\x1 x2 - f1 x1 x2 == f2 x1 x2)
instance Show (CmpFunction Integer) where
show f
| f `feq` CF (==) = ==
| f `feq` CF (/=) = /=
| f `feq` CF () =
| f `feq` CF (=) = =
| otherwise = Unknown function
This uses SmallCheck to figure out, with some degree of certainty,
whether two functions are equal.
Of course, Rice's theorem still holds, and the above instance is easy
to fool, but it still might be useful in some cases.
Roman
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[Haskell-cafe] Comparing functions
Hello Cafe, I have data CmpFunction a = CF (a - a - Bool) that contains comparing functions, like ==, , ..., and I'm trying to declare the Show instance for it like this instance Show (CmpFunction a) where show (CF (==)) = ==-- no good show f = case f of-- no good also CBF (==) - == _ - Other but compiler complains for both with This binding for `==' shadows the existing binding imported from `Prelude' at src/Main.hs:6:8-11 (and originally defined in `ghc-prim:GHC.Classes') Is it possible at all to compare two functions or how to solve this problem, to show some string for a specific function? br, vlatko ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing functions
On Thu, Jul 11, 2013 at 1:33 PM, Vlatko Basic vlatko.ba...@gmail.comwrote: data CmpFunction a = CF (a - a - Bool) that contains comparing functions, like ==, , ..., and I'm trying to declare the Show instance for it like this instance Show (CmpFunction a) where show (CF (==)) = ==-- no good show f = case f of-- no good also CBF (==) - == _ - Other but compiler complains for both with This binding for `==' shadows the existing binding imported from `Prelude' at src/Main.hs:6:8-11 (and originally defined in `ghc-prim:GHC.Classes') The problem here isn't quite what you think it is; (==) is not a constructor, therefore it is a *variable*. It's exactly the same problem as a = 5 -- ... foo a = 3 -- this does NOT compare with the previous value of a; it's identical to the next line! foo x = x Just as with the above, the normal way to do it would be to use a guard... but functions don't have an Eq instance, and *can't* have one. How do you meaningfully compare them? And for a typeclass function like (==), do you want (==) instantiated for Int to compare equal to (==) instantiated for Integer? Does a native-compiled function compare equal to an interpreted function? Remember referential transparency; the concept of comparing pointers used in some languages is not applicable to Haskell. -- brandon s allbery kf8nh sine nomine associates allber...@gmail.com ballb...@sinenomine.net unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing functions
On 07/11/2013 07:33 PM, Vlatko Basic wrote: Hello Cafe, I have data CmpFunction a = CF (a - a - Bool) that contains comparing functions, like ==, , ..., and I'm trying to declare the Show instance for it like this instance Show (CmpFunction a) where show (CF (==)) = ==-- no good show f = case f of-- no good also CBF (==) - == _ - Other but compiler complains for both with This binding for `==' shadows the existing binding imported from `Prelude' at src/Main.hs:6:8-11 (and originally defined in `ghc-prim:GHC.Classes') Yes, (==) is a variable name in a pattern. Hence you are creating a new definition for (==) bound to the constructor argument to CF, which hides the (==) defined within the Eq type class. Is it possible at all to compare two functions Function types are opaque and values do not have an identity. or how to solve this problem, to show some string for a specific function? br, vlatko You could store the string alongside the function inside the data type in some way. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing functions
Original Message
Subject: Re: [Haskell-cafe] Comparing functions
From: Brandon Allbery allber...@gmail.com
To: Vlatko Bašić vlatko.ba...@gmail.com
Cc: Haskell-Cafe haskell-cafe@haskell.org
Date: 11.07.2013 19:50
On Thu, Jul 11, 2013 at 1:33 PM, Vlatko Basic vlatko.ba...@gmail.com
wrote:
data
CmpFunction a = CF (a - a - Bool)
that contains comparing functions, like ==, ,
..., and I'm trying to declare the Show instance for it
like this
instance Show (CmpFunction a) where
show (CF (==)) = "== " -- no good
show f = case f of -- no
good also
CBF (==) - "=="
_ - "Other"
but compiler complains for both with
This binding for `==' shadows the existing binding
imported from `Prelude' at src/Main.hs:6:8-11
(and originally defined in
`ghc-prim:GHC.Classes')
The problem here isn't quite what you think it
is; (==) is not a constructor, therefore it is a
*variable*. It's exactly the same problem as
a = 5
-- ...
foo a = 3 -- this does NOT compare with
the previous value of "a"; it's identical to the next
line!
foo x = x
Hm, I thought it is a pattern match with constant, as in f ('a':xs)
==
Just as with the above, the normal way to do
it would be to use a guard... but functions don't have an
Eq instance, and *can't* have one. How do you meaningfully
compare them? And for a typeclass function like (==), do
you want (==) instantiated for Int to compare equal to
(==) instantiated for Integer? Does a native-compiled
function compare equal to an interpreted function?
Remember referential transparency; the concept of
comparing pointers used in some languages is not
applicable to Haskell.
--
brandon s allbery kf8nh
sine nomine associates
allber...@gmail.com
ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Re: [Haskell-cafe] Comparing functions
* Vlatko Basic vlatko.ba...@gmail.com [2013-07-11 19:33:38+0200]
Hello Cafe,
I have
data CmpFunction a = CF (a - a - Bool)
that contains comparing functions, like ==, , ..., and I'm trying
to declare the Show instance for it like this
instance Show (CmpFunction a) where
show (CF (==)) = ==-- no good
show f = case f of-- no good also
CBF (==) - ==
_ - Other
but compiler complains for both with
This binding for `==' shadows the existing binding
imported from `Prelude' at src/Main.hs:6:8-11
(and originally defined in `ghc-prim:GHC.Classes')
Is it possible at all to compare two functions or how to solve this
problem, to show some string for a specific function?
Depending on why you need that...
{-# LANGUAGE FlexibleContexts, UndecidableInstances, FlexibleInstances #-}
import Test.SmallCheck
import Test.SmallCheck.Series
import Test.SmallCheck.Drivers
import Control.Monad.Identity
import Data.Maybe
data CmpFunction a = CF (a - a - Bool)
feq :: (Show a, Serial Identity a) = CmpFunction a - CmpFunction a - Bool
feq (CF f1) (CF f2) =
isNothing $
runIdentity $
smallCheckM 10 (\x1 x2 - f1 x1 x2 == f2 x1 x2)
instance Show (CmpFunction Integer) where
show f
| f `feq` CF (==) = ==
| f `feq` CF (/=) = /=
| f `feq` CF () =
| f `feq` CF (=) = =
| otherwise = Unknown function
This uses SmallCheck to figure out, with some degree of certainty,
whether two functions are equal.
Of course, Rice's theorem still holds, and the above instance is easy
to fool, but it still might be useful in some cases.
Roman
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Re: [Haskell-cafe] Comparing functions
On Thu, Jul 11, 2013 at 10:50 AM, Brandon Allbery allber...@gmail.comwrote: ... but functions don't have an Eq instance, and *can't* have one. Not a general one that's interesting. There are two Eq instances that'll compile for all functions (not that it's advisable): instance Eq ((-) a b) where (==) _ _ = True instance Eq ((-) a b) where (==) _ _ = False You can't get more interesting in the general case, because you can't inspect the arguments. If you are okay with distinguishing solely by application you can get a little more interesting: instance (Bounded a, Enum a, Eq b) = Eq ((-) a b) where f == g = all (\ x - f x == g x) [minBound .. maxBound] *Main () == () True *Main () == (||) False Though I'm still not sure I'd say it's a *good idea*... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comparing functions
On Thu, Jul 11, 2013 at 2:11 PM, Vlatko Basic vlatko.ba...@gmail.comwrote:
The problem here isn't quite what you think it is; (==) is not a
constructor, therefore it is a *variable*. It's exactly the same problem as
a = 5
-- ...
foo a = 3 -- this does NOT compare with the previous value of a;
it's identical to the next line!
foo x = x
Hm, I thought it is a pattern match with constant, as in f ('a':xs) ==
I wonder what you'd make of this definition, then?
(*) `on` f = \x y - f x * f y
--
brandon s allbery kf8nh sine nomine associates
allber...@gmail.com ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net
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Re: [Haskell-cafe] Comparing functions
Original Message
Subject: Re: [Haskell-cafe] Comparing functions
From: Brandon Allbery allber...@gmail.com
To: Vlatko Bašić vlatko.ba...@gmail.com
Cc: Haskell-Cafe haskell-cafe@haskell.org
Date: 11.07.2013 20:45
On Thu, Jul 11, 2013 at 2:11 PM, Vlatko Basic vlatko.ba...@gmail.com
wrote:
The problem here isn't quite what you think
it is; (==) is not a constructor, therefore it
is a *variable*. It's exactly the same problem
as
a = 5
-- ...
foo a = 3 -- this does NOT compare
with the previous value of "a"; it's
identical to the next line!
foo x = x
Hm, I thought it is a pattern match with constant, as in
f ('a':xs) ==
I wonder what you'd make of this definition,
then?
(*)
`on` f =
\x y - f x * f y
According to the enlightenment above, I'd say (*) is a variable that
holds some function/operator that is applied on (f x) and (f y),
not the multiplication, right?
--
brandon s allbery kf8nh
sine nomine associates
allber...@gmail.com
ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
___
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Re: [Haskell-cafe] Comparing functions
On Thu, Jul 11, 2013 at 2:58 PM, Vlatko Basic vlatko.ba...@gmail.comwrote:
Hm, I thought it is a pattern match with constant, as in f ('a':xs) ==
I wonder what you'd make of this definition, then?
(*) `on` f = \x y - f x * f y
According to the enlightenment above, I'd say (*) is a variable that holds
some function/operator that is applied on (f x) and (f y), not the
multiplication, right?
Correct. But if it's a variable there, why would you expect it to be a
constant in a different pattern?
--
brandon s allbery kf8nh sine nomine associates
allber...@gmail.com ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonadhttp://sinenomine.net
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Re: [Haskell-cafe] Comparing functions
Original Message
Subject: Re: [Haskell-cafe] Comparing functions
From: Brandon Allbery allber...@gmail.com
To: Vlatko Bašić vlatko.ba...@gmail.com
Cc: Haskell-Cafe haskell-cafe@haskell.org
Date: 11.07.2013 21:03
On Thu, Jul 11, 2013 at 2:58 PM, Vlatko Basic vlatko.ba...@gmail.com
wrote:
Hm, I thought it is a pattern match with
constant, as in f ('a':xs) ==
I wonder what you'd make of this
definition, then?
(*) `on` f = \x y - f x * f y
According to the enlightenment above, I'd say (*) is a
variable that holds some function/operator that is
applied on (f x) and (f y), not the multiplication,
right?
Correct. But if it's a variable there, why
would you expect it to be a constant in a different
pattern?
Well, it is confusing that an operator can be a variable. I must get
a habit to understand the meaning by the site, not by the looks.
Thanks for your answers.
--
brandon s allbery kf8nh
sine nomine associates
allber...@gmail.com
ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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