Flexible instances

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Improving MTL instances (was:...

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George Pollard

14 Oct 2008 14 Oct '08

10:20 p.m.

I'm a little confused. Why is this allowed:

...

data Blah = Blah

instance Eq Blah where x == y = True

But not this:

...

class Stringable a where toString :: a -> String

instance Stringable [Char] where toString = id

(Resulting in:)

...

Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

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Derek Elkins

14 Oct 14 Oct

10:32 p.m.

On Tue, 2008-10-14 at 19:20 -0700, George Pollard wrote:

...

I'm a little confused. Why is this allowed:

...
data Blah = Blah

instance Eq Blah where x == y = True

But not this:

...
class Stringable a where toString :: a -> String

instance Stringable [Char] where toString = id

(Resulting in:)

...
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

Blah = T for [Char], T = [] and a1 = Char where it should be a variable. Why this is an error is basically because the Report says so.

Hi, There is some discussion about the different design choices relevant for Haskell's class system in the following paper: "Type classes: exploring the design space" Simon Peyton Jones, Mark Jones, Erik Meijer Presented at the 1997 Haskell Workshop. Section 4.5 discusses options related to the restrictions on the instance heads. -Iavor On Tue, Oct 14, 2008 at 7:32 PM, Derek Elkins wrote:

...

On Tue, 2008-10-14 at 19:20 -0700, George Pollard wrote:

...
I'm a little confused. Why is this allowed:

...
data Blah = Blah

instance Eq Blah where x == y = True

But not this:

...
class Stringable a where toString :: a -> String

instance Stringable [Char] where toString = id

(Resulting in:)

...
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

Blah = T

for [Char], T = [] and a1 = Char where it should be a variable.

Why this is an error is basically because the Report says so.

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Luke Palmer

10:37 p.m.

On Tue, Oct 14, 2008 at 8:20 PM, George Pollard wrote:

...

...
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

If you're stuck, this is a fairly arbitrary limitation and is easily generalized. GHC, at least, implements the FlexibleInstances extension. Put {-# LANGUAGE FlexibleInstances #-} At the top of the file and that will be an acceptable instance. Luke

Albert Y. C. Lai

11:51 p.m.

George Pollard wrote:

...

I'm a little confused. Why is this allowed:

...
instance Eq Blah where x == y = True

But not this:

...
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

Just in case: n=0 for "instance Eq Blah", i.e., "T a1 ... an" becomes "T".

Bulat Ziganshin

15 Oct 15 Oct

6:30 a.m.

New subject: Re[2]: Flexible instances

Hello Albert, Wednesday, October 15, 2008, 7:51:06 AM, you wrote:

...

...
...
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables*

...

Just in case: n=0 for "instance Eq Blah", i.e., "T a1 ... an" becomes "T".

and [Char] = [] Char, where Char isn't type variable but constant -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

Ryan Ingram

3:16 a.m.

So, the Haskell98 solution to this is: class StringableList a where listToString :: [a] -> String -- now [a] is of the proper form; T = [], a is a type variable instance StringableList a => Stringable [a] where toString = listToString -- now to make an instance for Stringable [Char] -- we just make an instance for StringableList Char instance StringableList Char where listToString = id I think "FlexibleInstances" just makes the compiler jump through these hoops instead of you. -- ryan On Wed, Oct 15, 2008 at 3:20 AM, George Pollard wrote:

...

I'm a little confused. Why is this allowed:

...
data Blah = Blah

instance Eq Blah where x == y = True

But not this:

...
class Stringable a where toString :: a -> String

instance Stringable [Char] where toString = id

(Resulting in:)

...
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

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Anton Tayanovskyy

5:31 a.m.

Thanks for the portable solution. I'd also like to know how is the following different from -XFlexibleInstances with [Char]? Stronger, weaker, same thing? {-# OPTIONS -XTypeSynonymInstances #-} class Stringable a where toString :: a -> String instance Stringable String where toString = id --A On Wed, Oct 15, 2008 at 10:16 AM, Ryan Ingram wrote:

...

So, the Haskell98 solution to this is:

class StringableList a where listToString :: [a] -> String

-- now [a] is of the proper form; T = [], a is a type variable instance StringableList a => Stringable [a] where toString = listToString

-- now to make an instance for Stringable [Char] -- we just make an instance for StringableList Char instance StringableList Char where listToString = id

I think "FlexibleInstances" just makes the compiler jump through these hoops instead of you.

-- ryan

On Wed, Oct 15, 2008 at 3:20 AM, George Pollard wrote:

...
I'm a little confused. Why is this allowed:

...
data Blah = Blah

instance Eq Blah where x == y = True

But not this:

...
class Stringable a where toString :: a -> String

instance Stringable [Char] where toString = id

(Resulting in:)

...
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

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George Pollard

6:28 a.m.

Thanks to all that replied (Derek & Ryan for the major understanding, Albert & Luke also)*. I guess I was getting confused with the error message:

...

(All instance types must be of the form (T a1 ... an)

I was interpreting this with Stringable/Enum as T and [Char]/Blah as a1. Now I have clarity! I shall have to follow up Iavor's lead on why this *is*. Thanks! *[Silly email reply system; discussions on the internet should form an acyclic directed graph, not a tree! This, incidentally, is why I'm replying to myself.]

Henning Thielemann

5:14 p.m.

George Pollard schrieb:

...

I'm a little confused. Why is this allowed:

...
data Blah = Blah

instance Eq Blah where x == y = True

But not this:

...
class Stringable a where toString :: a -> String

instance Stringable [Char] where toString = id

(Resulting in:)

...
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'

'Blah' isn't a type variable, is it? Is my brain just not working right today?

http://www.haskell.org/haskellwiki/List_instance

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participants (9)

Albert Y. C. Lai
Anton Tayanovskyy
Bulat Ziganshin
Derek Elkins
George Pollard
Henning Thielemann
Iavor Diatchki
Luke Palmer
Ryan Ingram