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Today's Topics:
1. Ambiguous type error: multiparam class + type alias (Baa)
2. Re: Ambiguous type error: multiparam class + type alias
(David McBride)
3. Re: Ambiguous type error: multiparam class + type alias (Baa)
4. Re: Ambiguous type error: multiparam class + type alias
(Sylvain Henry)
5. Re: Ambiguous type error: multiparam class + type alias (Baa)
----------------------------------------------------------------------
Message: 1
Date: Wed, 27 Sep 2017 18:50:38 +0300
From: Baa <aqua...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] Ambiguous type error: multiparam class +
type alias
Message-ID: <20170927185038.44cde600@Pavel>
Content-Type: text/plain; charset=UTF-8
Hello, List!
The further - the more interesting... For example, I want own `Show` analogue,
which will be parameterized with type meaning "context"/"reason". So, to
represent
title of the value I will call:
repr x::TitleRepr
or simple `Show`:
repr x::ShowRepr
or
repr x::Repr AsShow
etc. I try:
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE MultiParamTypeClasses #-}
data AsShow
data AsTitle
type Repr a = String
class ReprC a b where
repr :: a -> Repr b
instance ReprC Int AsTitle where
repr n = "a number " ++ show n
main = do
let n = 5 :: Int
print $ (repr n :: Repr AsTitle)
and sure I get error:
• Ambiguous type variable ‘b0’ arising from a use of ‘repr’
prevents the constraint ‘(ReprC Int b0)’ from being solved.
Probable fix: use a type annotation to specify what ‘b0’ should be.
These potential instance exist:
instance ReprC Int AsTitle
-- Defined at .../.stack-work/intero/intero31144sPV.hs:12:10
• In the second argument of ‘($)’, namely
‘(repr n :: Repr AsTitle)’
In a stmt of a 'do' block: print $ (repr n :: Repr AsTitle)
In the expression:
do { let n = ...;
print $ (repr n :: Repr AsTitle) } (intero)
Sure, I can use as `Repr` not type-alias but `newtype`. But in this
case I will need additional call (show/runRepr/coerce/etc.):
coerce $ (repr x::AsTitle)
So, what is the reason that GHCI is see that `repr n` is `::Repr AsTitle`
(AsTitle!!) and says me that `b0` is ambigous?! It should know
what concreate `repr` I mean! :) If I use `newtype` - no problem, so I
suppose problem is in the type alias. It's not sterling type for such
goal, right?
Another question is: how to accomplish such goal, i.e. without to
make additional call (coerce/show/runRepr/etc) when `repr` will return
`String`, wrapped in newtype?
PS. Execuse such silly and training example. Actually, I planned to made
such class and to use it instead of `Show`. Sure, it can be splitted to
several classed (my current state) but.. example is to learn Haskell
and to understand my errors...
===
Best regards, Paul
------------------------------
Message: 2
Date: Wed, 27 Sep 2017 12:58:32 -0400
From: David McBride <toa...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Ambiguous type error: multiparam
class + type alias
Message-ID:
<can+tr431zb7ojkkyhe_zew+0gsiuannsjcz5dp2hcrarowe...@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
The reason is because 'type' is a type alias that changes nothing code
wise, it is merely a visual clue to the reader of code that these two
types are the same. You could replace 'Repr AsTitle' with 'String'
and you would get the exact same error.
That said I do think you can do what you want with type families, but
I'm not having luck giving you a complete solution at this time. This
would be a good question to ask on stackoverflow if no one here gives
a satisfactory answer. Here is the code I had that doesn't quite
work, although I'm not sure why. Maybe you can figure it out. I
would love to know.
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}
data AsShow
data AsTitle
class Repr a b where
type ReprC a b :: *
repr :: a -> ReprC a b
instance Repr Int AsTitle where
type ReprC Int AsTitle = String
repr n = show n
main = do
let n = 5 :: Int
print $ (repr n)
On Wed, Sep 27, 2017 at 11:50 AM, Baa <aqua...@gmail.com> wrote:
> Hello, List!
>
> The further - the more interesting... For example, I want own `Show` analogue,
> which will be parameterized with type meaning "context"/"reason". So, to
> represent
> title of the value I will call:
>
> repr x::TitleRepr
>
> or simple `Show`:
>
> repr x::ShowRepr
>
> or
>
> repr x::Repr AsShow
>
> etc. I try:
>
> {-# LANGUAGE AllowAmbiguousTypes #-}
> {-# LANGUAGE MultiParamTypeClasses #-}
>
> data AsShow
> data AsTitle
>
> type Repr a = String
>
> class ReprC a b where
> repr :: a -> Repr b
>
> instance ReprC Int AsTitle where
> repr n = "a number " ++ show n
>
> main = do
> let n = 5 :: Int
> print $ (repr n :: Repr AsTitle)
>
> and sure I get error:
>
> • Ambiguous type variable ‘b0’ arising from a use of ‘repr’
> prevents the constraint ‘(ReprC Int b0)’ from being solved.
> Probable fix: use a type annotation to specify what ‘b0’ should be.
> These potential instance exist:
> instance ReprC Int AsTitle
> -- Defined at .../.stack-work/intero/intero31144sPV.hs:12:10
> • In the second argument of ‘($)’, namely
> ‘(repr n :: Repr AsTitle)’
> In a stmt of a 'do' block: print $ (repr n :: Repr AsTitle)
> In the expression:
> do { let n = ...;
> print $ (repr n :: Repr AsTitle) } (intero)
>
> Sure, I can use as `Repr` not type-alias but `newtype`. But in this
> case I will need additional call (show/runRepr/coerce/etc.):
>
> coerce $ (repr x::AsTitle)
>
> So, what is the reason that GHCI is see that `repr n` is `::Repr AsTitle`
> (AsTitle!!) and says me that `b0` is ambigous?! It should know
> what concreate `repr` I mean! :) If I use `newtype` - no problem, so I
> suppose problem is in the type alias. It's not sterling type for such
> goal, right?
>
> Another question is: how to accomplish such goal, i.e. without to
> make additional call (coerce/show/runRepr/etc) when `repr` will return
> `String`, wrapped in newtype?
>
>
> PS. Execuse such silly and training example. Actually, I planned to made
> such class and to use it instead of `Show`. Sure, it can be splitted to
> several classed (my current state) but.. example is to learn Haskell
> and to understand my errors...
>
>
> ===
> Best regards, Paul
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
Message: 3
Date: Thu, 28 Sep 2017 10:17:23 +0300
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Ambiguous type error: multiparam
class + type alias
Message-ID: <20170928101723.0bbb02ba@Pavel>
Content-Type: text/plain; charset=UTF-8
David, hello again!
Interesting is that class with one parameter is fine:
class Repr a where
type ReprAs a
reprx :: a -> ReprAs a
instance Repr Int where
type ReprAs Int = Int
reprx n = n
main = do
let n = 5 :: Int
print $ (reprx n)
but when I added 2nd param, I get ambogouse error again:
class Repr a b where
type ReprAs a b
repr :: a -> ReprAs a b
instance Repr Int AsTitle where
type ReprAs Int AsTitle = String
repr n = show n
main = do
let n = 5 :: Int
print $ (repr n::ReprAs Int AsTitle)
which looks the same as with type-alias: GHC says:
30 12 error error:
• Couldn't match type ‘ReprAs Int b0’ with ‘String’
Expected type: ReprAs Int AsTitle
Actual type: ReprAs Int b0
The type variable ‘b0’ is ambiguous
• In the second argument of ‘($)’, namely
‘(repr n :: ReprAs Int AsTitle)’
In a stmt of a 'do' block: print $ (repr n :: ReprAs Int AsTitle)
In the expression:
do { let n = ...;
print $ (repr n :: ReprAs Int AsTitle) } (intero)
looks that `type ReprAs Int AsTitle` is treating type-alias and GHC can
not match resulting `String` with `ReprAs Int AsTitle`.
As I understand, the root of the problem is that class parameters are
not the same as in results' parameters (contraposition types) - they
turn out to be free/unbound (execuse my English).
For example, to avoid this problem in the class I added extension:
{-# LANGUAGE AllowAmbiguousTypes #-}
I don't know is a way in Haskell to say to reuse already bound class
parameters in "methods" bodies... I found this:
https://stackoverflow.com/questions/4174187/reading-and-representing-input-which-specifies-the-data-type-to-use/4174266#4174266
Another variant which is compiling (w/ func-deps):
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
data AsShow
data AsTitle
type ReprAs a = String
class Repr a b | a -> b where
repr :: a -> ReprAs b
instance Repr Int AsTitle where
repr n = "a number '" ++ show n ++ "'"
-- instance Repr Int AsShow where
-- repr n = "a number '" ++ show n ++ "'"
main = do
let n = 5 :: Int
print $ (repr n::ReprAs AsTitle)
print $ (repr n::ReprAs AsShow)
but due to `a -> b` it's impossible to instantiate another `Repr Int`!
So, I think soultion is in:
- func deps
- or type families/associative types
But unfortunately my knowledge of Haskell is limited to find it. I see
only that I need to add another param to func dep of class - to
"extend" dependency which allows to determine result type not only on
one input argument's type...
Thanks David!
===
Best regards, Paul
> The reason is because 'type' is a type alias that changes nothing code
> wise, it is merely a visual clue to the reader of code that these two
> types are the same. You could replace 'Repr AsTitle' with 'String'
> and you would get the exact same error.
>
> That said I do think you can do what you want with type families, but
> I'm not having luck giving you a complete solution at this time. This
> would be a good question to ask on stackoverflow if no one here gives
> a satisfactory answer. Here is the code I had that doesn't quite
> work, although I'm not sure why. Maybe you can figure it out. I
> would love to know.
>
> {-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}
>
> data AsShow
> data AsTitle
>
> class Repr a b where
> type ReprC a b :: *
> repr :: a -> ReprC a b
>
> instance Repr Int AsTitle where
> type ReprC Int AsTitle = String
> repr n = show n
>
> main = do
> let n = 5 :: Int
> print $ (repr n)
>
> On Wed, Sep 27, 2017 at 11:50 AM, Baa <aqua...@gmail.com> wrote:
> > Hello, List!
> >
> > The further - the more interesting... For example, I want own
> > `Show` analogue, which will be parameterized with type meaning
> > "context"/"reason". So, to represent title of the value I will call:
> >
> > repr x::TitleRepr
> >
> > or simple `Show`:
> >
> > repr x::ShowRepr
> >
> > or
> >
> > repr x::Repr AsShow
> >
> > etc. I try:
> >
> > {-# LANGUAGE AllowAmbiguousTypes #-}
> > {-# LANGUAGE MultiParamTypeClasses #-}
> >
> > data AsShow
> > data AsTitle
> >
> > type Repr a = String
> >
> > class ReprC a b where
> > repr :: a -> Repr b
> >
> > instance ReprC Int AsTitle where
> > repr n = "a number " ++ show n
> >
> > main = do
> > let n = 5 :: Int
> > print $ (repr n :: Repr AsTitle)
> >
> > and sure I get error:
> >
> > • Ambiguous type variable ‘b0’ arising from a use of ‘repr’
> > prevents the constraint ‘(ReprC Int b0)’ from being solved.
> > Probable fix: use a type annotation to specify what ‘b0’
> > should be. These potential instance exist:
> > instance ReprC Int AsTitle
> > -- Defined
> > at .../.stack-work/intero/intero31144sPV.hs:12:10 • In the second
> > argument of ‘($)’, namely ‘(repr n :: Repr AsTitle)’
> > In a stmt of a 'do' block: print $ (repr n :: Repr AsTitle)
> > In the expression:
> > do { let n = ...;
> > print $ (repr n :: Repr AsTitle) } (intero)
> >
> > Sure, I can use as `Repr` not type-alias but `newtype`. But in this
> > case I will need additional call (show/runRepr/coerce/etc.):
> >
> > coerce $ (repr x::AsTitle)
> >
> > So, what is the reason that GHCI is see that `repr n` is `::Repr
> > AsTitle` (AsTitle!!) and says me that `b0` is ambigous?! It should
> > know what concreate `repr` I mean! :) If I use `newtype` - no
> > problem, so I suppose problem is in the type alias. It's not
> > sterling type for such goal, right?
> >
> > Another question is: how to accomplish such goal, i.e. without to
> > make additional call (coerce/show/runRepr/etc) when `repr` will
> > return `String`, wrapped in newtype?
> >
> >
> > PS. Execuse such silly and training example. Actually, I planned to
> > made such class and to use it instead of `Show`. Sure, it can be
> > splitted to several classed (my current state) but.. example is to
> > learn Haskell and to understand my errors...
> >
> >
> > ===
> > Best regards, Paul
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
Message: 4
Date: Thu, 28 Sep 2017 11:12:35 +0200
From: Sylvain Henry <sylv...@haskus.fr>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Ambiguous type error: multiparam
class + type alias
Message-ID: <4041522b-8d46-f53f-d3a5-632999cc8...@haskus.fr>
Content-Type: text/plain; charset=utf-8; format=flowed
Hi,
The issue is that (Repr a ~ Repr b) doesn't imply (a ~ b). Indeed:
forall a b. Repr a ~ String ~ Repr b
So given the type of `repr n`:
repr n :: forall b. ReprC Int b => Repr b
When you write:
repr n :: Repr AsTitle
You only add a constraint as follows:
repr n :: forall b. (Repr b ~ Repr AsTitle, ReprC Int b) => Repr b
But as we have seen, it doesn't imply (b ~ AsTitle) and b remains ambiguous.
A solution is to fix b explicitly with a type application:
{-# LANGUAGE TypeApplications #-}
...
repr @_ @AsTitle n
Best regards,
Sylvain
On 27/09/2017 17:50, Baa wrote:
> Hello, List!
>
> The further - the more interesting... For example, I want own `Show` analogue,
> which will be parameterized with type meaning "context"/"reason". So, to
> represent
> title of the value I will call:
>
> repr x::TitleRepr
>
> or simple `Show`:
>
> repr x::ShowRepr
>
> or
>
> repr x::Repr AsShow
>
> etc. I try:
>
> {-# LANGUAGE AllowAmbiguousTypes #-}
> {-# LANGUAGE MultiParamTypeClasses #-}
>
> data AsShow
> data AsTitle
>
> type Repr a = String
>
> class ReprC a b where
> repr :: a -> Repr b
>
> instance ReprC Int AsTitle where
> repr n = "a number " ++ show n
>
> main = do
> let n = 5 :: Int
> print $ (repr n :: Repr AsTitle)
>
> and sure I get error:
>
> • Ambiguous type variable ‘b0’ arising from a use of ‘repr’
> prevents the constraint ‘(ReprC Int b0)’ from being solved.
> Probable fix: use a type annotation to specify what ‘b0’ should be.
> These potential instance exist:
> instance ReprC Int AsTitle
> -- Defined at .../.stack-work/intero/intero31144sPV.hs:12:10
> • In the second argument of ‘($)’, namely
> ‘(repr n :: Repr AsTitle)’
> In a stmt of a 'do' block: print $ (repr n :: Repr AsTitle)
> In the expression:
> do { let n = ...;
> print $ (repr n :: Repr AsTitle) } (intero)
>
> Sure, I can use as `Repr` not type-alias but `newtype`. But in this
> case I will need additional call (show/runRepr/coerce/etc.):
>
> coerce $ (repr x::AsTitle)
>
> So, what is the reason that GHCI is see that `repr n` is `::Repr AsTitle`
> (AsTitle!!) and says me that `b0` is ambigous?! It should know
> what concreate `repr` I mean! :) If I use `newtype` - no problem, so I
> suppose problem is in the type alias. It's not sterling type for such
> goal, right?
>
> Another question is: how to accomplish such goal, i.e. without to
> make additional call (coerce/show/runRepr/etc) when `repr` will return
> `String`, wrapped in newtype?
>
>
> PS. Execuse such silly and training example. Actually, I planned to made
> such class and to use it instead of `Show`. Sure, it can be splitted to
> several classed (my current state) but.. example is to learn Haskell
> and to understand my errors...
>
>
> ===
> Best regards, Paul
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
Message: 5
Date: Thu, 28 Sep 2017 13:25:09 +0300
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Ambiguous type error: multiparam
class + type alias
Message-ID: <20170928132509.0ffcc173@Pavel>
Content-Type: text/plain; charset=UTF-8
Hello, Sylvain.
Your solution assumes that I need to pass `@_ @AsTitle` anywhere where I
call `repr`? So, instead of `repr n::AsTitle` or `repr n::Something AsTitle`
(depends on implementation) I'll write `repr @_ @AsTitle n`, right?
> You only add a constraint as follows:
> repr n :: forall b. (Repr b ~ Repr AsTitle, ReprC Int b) => Repr b
Yes... So, `b` is unbound/free type param. But I can bind it with func.
deps, yes?
===
Best regards, Paul
>
> But as we have seen, it doesn't imply (b ~ AsTitle) and b remains
> ambiguous.
>
> A solution is to fix b explicitly with a type application:
>
> {-# LANGUAGE TypeApplications #-}
> ...
> repr @_ @AsTitle n
>
>
> Best regards,
> Sylvain
>
>
> On 27/09/2017 17:50, Baa wrote:
> > Hello, List!
> >
> > The further - the more interesting... For example, I want own
> > `Show` analogue, which will be parameterized with type meaning
> > "context"/"reason". So, to represent title of the value I will call:
> >
> > repr x::TitleRepr
> >
> > or simple `Show`:
> >
> > repr x::ShowRepr
> >
> > or
> >
> > repr x::Repr AsShow
> >
> > etc. I try:
> >
> > {-# LANGUAGE AllowAmbiguousTypes #-}
> > {-# LANGUAGE MultiParamTypeClasses #-}
> >
> > data AsShow
> > data AsTitle
> >
> > type Repr a = String
> >
> > class ReprC a b where
> > repr :: a -> Repr b
> >
> > instance ReprC Int AsTitle where
> > repr n = "a number " ++ show n
> >
> > main = do
> > let n = 5 :: Int
> > print $ (repr n :: Repr AsTitle)
> >
> > and sure I get error:
> >
> > • Ambiguous type variable ‘b0’ arising from a use of ‘repr’
> > prevents the constraint ‘(ReprC Int b0)’ from being solved.
> > Probable fix: use a type annotation to specify what ‘b0’
> > should be. These potential instance exist:
> > instance ReprC Int AsTitle
> > -- Defined
> > at .../.stack-work/intero/intero31144sPV.hs:12:10 • In the second
> > argument of ‘($)’, namely ‘(repr n :: Repr AsTitle)’
> > In a stmt of a 'do' block: print $ (repr n :: Repr AsTitle)
> > In the expression:
> > do { let n = ...;
> > print $ (repr n :: Repr AsTitle) } (intero)
> >
> > Sure, I can use as `Repr` not type-alias but `newtype`. But in this
> > case I will need additional call (show/runRepr/coerce/etc.):
> >
> > coerce $ (repr x::AsTitle)
> >
> > So, what is the reason that GHCI is see that `repr n` is `::Repr
> > AsTitle` (AsTitle!!) and says me that `b0` is ambigous?! It should
> > know what concreate `repr` I mean! :) If I use `newtype` - no
> > problem, so I suppose problem is in the type alias. It's not
> > sterling type for such goal, right?
> >
> > Another question is: how to accomplish such goal, i.e. without to
> > make additional call (coerce/show/runRepr/etc) when `repr` will
> > return `String`, wrapped in newtype?
> >
> >
> > PS. Execuse such silly and training example. Actually, I planned to
> > made such class and to use it instead of `Show`. Sure, it can be
> > splitted to several classed (my current state) but.. example is to
> > learn Haskell and to understand my errors...
> >
> >
> > ===
> > Best regards, Paul
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
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