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Today's Topics:
1. MultiParamTypeClasses confusion (Derek McLoughlin)
2. Re: MultiParamTypeClasses confusion (Brandon Allbery)
3. Re: MultiParamTypeClasses confusion (Derek McLoughlin)
4. Questions too basic for this mailing list? (Kim-Ee Yeoh)
----------------------------------------------------------------------
Message: 1
Date: Mon, 10 Mar 2014 12:59:04 +0000
From: Derek McLoughlin <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] MultiParamTypeClasses confusion
Message-ID:
<CAAw9fm=icejvp5lkw-6ie8ctr_vr0gcwv073l0logfkvrfx...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Hi,
In Chapter 6 of "Beginning Haskell" by Apress there's a couple of
classes introduced for vectors and things that can be vectorized (not
related to Data.Vector)
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
class Vector v where
distance :: v -> v -> Double
instance Vector (Double, Double) where
distance (a,b) (c,d) = sqrt $ (c - a) * (c - a) + (d - b) * (d - b)
class Vectorizable e v where
toVector :: e -> v
instance Vectorizable (Double, Double) (Double, Double) where
toVector = id
x = 1.0 :: Double
y = 10.0 :: Double
While I understand how to use the "distance" function:
ghci> distance (x, x) (y, y)
12.727922061357855
... and I can see how "toVector" is used in their code
ghci> distance (x, x) $ toVector (y, y)
12.727922061357855
I don't understand why this doesn't work:
ghci> let z = toVector (y, y)
interactive>:32:9:
No instance for (Vectorizable (Double, Double) v0)
arising from a use of `toVector'
The type variable `v0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there is a potential instance available:
instance Vectorizable (Double, Double) (Double, Double)
-- Defined at vector.hs:13:10
Possible fix:
add an instance declaration for (Vectorizable (Double, Double) v0)
In the expression: toVector (y, y)
In an equation for `z': z = toVector (y, y)
It seems odd that "toVector" works when used as an argument to
"distance" but not when used in a let expression.
Can anyone explain?
Derek.
------------------------------
Message: 2
Date: Mon, 10 Mar 2014 09:12:13 -0400
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] MultiParamTypeClasses confusion
Message-ID:
<cakfcl4uyuzyutq050wt7y5dk8tdo1kjlmvo60-gkstaivqd...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Mon, Mar 10, 2014 at 8:59 AM, Derek McLoughlin <
[email protected]> wrote:
> ghci> distance (x, x) $ toVector (y, y)
> 12.727922061357855
>
> I don't understand why this doesn't work:
>
> ghci> let z = toVector (y, y)
> interactive>:32:9:
> No instance for (Vectorizable (Double, Double) v0)
> arising from a use of `toVector'
> The type variable `v0' is ambiguous
> Possible fix: add a type signature that fixes these type variable(s)
> Note: there is a potential instance available:
> instance Vectorizable (Double, Double) (Double, Double)
> -- Defined at vector.hs:13:10
> Possible fix:
> add an instance declaration for (Vectorizable (Double, Double) v0)
> In the expression: toVector (y, y)
> In an equation for `z': z = toVector (y, y)
>
> It seems odd that "toVector" works when used as an argument to
> "distance" but not when used in a let expression.
>
> Can anyone explain?
>
When you use them together, Haskell can infer the correct type for `v0`
from the inferred type of `distance` by applying defaulting: the type of
`sqrt` introduces a constraint that is subject to defaulting. If you
separate them, it can no longer determine a concrete type for your `let`,
as multiparameter type classes are not subject to defaulting (and can't
be). Also, Haskell cannot conclude from the existence of a single possible
instance that it should use that instance: the meaning of your expression
would then change if you imported a module which defined a new instance.
The requirement for a concrete type comes from the monomorphism
restriction, which is applied to your `let` because `z` has no parameters.
http://www.haskell.org/haskellwiki/Monomorphism_restriction
Many people turn off the monomorphism restriction in ghci to avoid this
kind of issue.
:set -XNoMonomoprhismRestriction
which you can put in ~/.ghci so that it is the default for new ghci
sessions.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 3
Date: Mon, 10 Mar 2014 13:21:52 +0000
From: Derek McLoughlin <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] MultiParamTypeClasses confusion
Message-ID:
<CAAw9fmm8gn=dx5ayzdmdpwsozvxjfzqbrmejszusqts0jsn...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Thanks for that. I see that I can now use:
ghci> let z = toVector (y, y) :: (Double, Double)
ghci> z
(10.0,10.0)
On 10 March 2014 13:12, Brandon Allbery <[email protected]> wrote:
> On Mon, Mar 10, 2014 at 8:59 AM, Derek McLoughlin
> <[email protected]> wrote:
>>
>> ghci> distance (x, x) $ toVector (y, y)
>> 12.727922061357855
>>
>> I don't understand why this doesn't work:
>>
>> ghci> let z = toVector (y, y)
>> interactive>:32:9:
>> No instance for (Vectorizable (Double, Double) v0)
>> arising from a use of `toVector'
>> The type variable `v0' is ambiguous
>> Possible fix: add a type signature that fixes these type variable(s)
>> Note: there is a potential instance available:
>> instance Vectorizable (Double, Double) (Double, Double)
>> -- Defined at vector.hs:13:10
>> Possible fix:
>> add an instance declaration for (Vectorizable (Double, Double) v0)
>> In the expression: toVector (y, y)
>> In an equation for `z': z = toVector (y, y)
>>
>> It seems odd that "toVector" works when used as an argument to
>> "distance" but not when used in a let expression.
>>
>> Can anyone explain?
>
>
> When you use them together, Haskell can infer the correct type for `v0` from
> the inferred type of `distance` by applying defaulting: the type of `sqrt`
> introduces a constraint that is subject to defaulting. If you separate them,
> it can no longer determine a concrete type for your `let`, as multiparameter
> type classes are not subject to defaulting (and can't be). Also, Haskell
> cannot conclude from the existence of a single possible instance that it
> should use that instance: the meaning of your expression would then change
> if you imported a module which defined a new instance.
>
> The requirement for a concrete type comes from the monomorphism restriction,
> which is applied to your `let` because `z` has no parameters.
>
> http://www.haskell.org/haskellwiki/Monomorphism_restriction
>
> Many people turn off the monomorphism restriction in ghci to avoid this kind
> of issue.
>
> :set -XNoMonomoprhismRestriction
>
> which you can put in ~/.ghci so that it is the default for new ghci
> sessions.
>
> --
> brandon s allbery kf8nh sine nomine associates
> [email protected] [email protected]
> unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 4
Date: Tue, 11 Mar 2014 17:04:37 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Questions too basic for this mailing
list?
Message-ID:
<capy+zdqs_qs1t8owhzm5s+hrycfwvzcgu84yt_k3dhzz8dr...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
One of the attributes of the haskell community that I take pride in is the
balance between democracy and upholding standards.
High technical standards such as correctness, ease-of-reasoning, brevity
are valued, as well as high community standards such as humor, empathy,
addressing the other with dignity.
All the above, coupled with an openness to the world-at-large, is the kit
and kaboodle of the accidental haskeller.
So when the response to a newcomer boils down to "take your basic questions
elsewhere", I wonder about these values and I wonder about the community's
openness.
I for one regret losing the opportunity of exploring very interesting
questions about syntax, even elementary syntax, a common obstacle faced by
many.
Consider how Haskell syntax was designed in a rare confluence of events
requiring tremendous buy-in (and patience, and empathy, and openness) from
a superficially similar but actually a very diverse group with vigorous,
opposing opinions.
In an environment that turns inimical to such basic, foundational values --
whither progress?
I feel the loss deeply.
-- Kim-Ee
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