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List instance

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Current revision (13:38, 11 December 2010) (edit) (undo)
(custom newtype is a nice solution in some cases)
 
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If I have a type class for conversion to a type X:
+
If I have a type class for conversion to a type <hask>X</hask>:
-
<haskell>
+
<haskell>
class C a where
class C a where
toX :: a -> X
toX :: a -> X
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</haskell>
</haskell>
-
but not for Strings, given that they are a synonym for [Char]. Hence:
+
but not for Strings, given that they are a synonym for <hask>[Char]</hask>. Hence:
<haskell>
<haskell>
Line 35: Line 35:
</code>
</code>
-
Is there some type class cleverness that can make this work in haskell
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Is there some type class cleverness that can make this work in Haskell 98?
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98? I can create a new wrapper type for strings:
+
-
<haskell>
+
I'm aware of the approach taken by class <hask>Show</hask> in the prelude,
-
newtype StringWrap = StringWrap String
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which adds a extra method to the class:
-
</haskell>
+
-
 
+
-
and write an instance for that, but then I'll have to litter my code
+
-
with calls to this constructor.
+
-
 
+
-
I'm aware of the approach taken by class Show in the prelude, which
+
-
adds a extra method to the class:
+
-
+
<haskell>
<haskell>
class C a where
class C a where
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== Answer ==
== Answer ==
-
The trick in the Prelude is that <hask>listToX</hask> has a default implementation.
+
First you have to find out, what you really want to do.
-
If this is not possible in your application then introduce a new class like
+
=== Define a custom type ===
 +
If you do not need list functions, but e.g. just want to use <hask>String</hask> values as identifiers,
 +
then I advise to just define a custom type:
 +
<haskell>
 +
newtype Identifier = Identifier String
 +
</haskell>
 +
 +
and write the instances you need for that type.
 +
 +
=== Keep working with the list (or container) type ===
 +
 +
If you need list functions,
 +
then the wrapper approach means that you have to litter your code with calls to the wrapper constructor.
 +
In this case you should stick to the original composed type.
 +
The trick in the Prelude for the <hask>Show</hask> class is,
 +
that <hask>listToX</hask> has a default implementation.
 +
 +
If this is not possible in your application then introduce a new class like
<haskell>
<haskell>
class Element a where
class Element a where
listToX :: [a] -> X
listToX :: [a] -> X
</haskell>
</haskell>
-
 
and define instances like
and define instances like
-
 
<haskell>
<haskell>
instance Element Char where
instance Element Char where
Line 78: Line 84:
</haskell>
</haskell>
.
.
 +
 +
More generally, you can introduce a new class like
 +
<haskell>
 +
class IsChar a where
 +
fromChar :: Char -> a
 +
toChar :: a -> Char
 +
instance IsChar Char where
 +
fromChar = id
 +
toChar = id
 +
</haskell>
 +
and then whenever you want to use the "<hask>a</hask>" type as a <hask>Char</hask>,
 +
you just convert it from or to <hask>Char</hask> with <hask>fromChar</hask> or <hask>toChar</hask>, respectively.
 +
 +
=== FlexibleInstances ===
 +
 +
If none of the above solution is appropriate for you,
 +
then you may choose the <code>FlexibleInstances</code> language extension
 +
that is available in GHC and Hugs.
 +
See http://www.haskell.org/ghc/docs/6.8-latest/html/users_guide/type-class-extensions.html#instance-rules
 +
 +
<haskell>
 +
{-# LANGUAGE FlexibleInstances #-}
 +
instance C [Char] where toX = ...
 +
</haskell>
== Source ==
== Source ==
http://www.haskell.org/pipermail/haskell-cafe/2007-May/025742.html
http://www.haskell.org/pipermail/haskell-cafe/2007-May/025742.html
 +
 +
[[Category:FAQ]]
 +
[[Category:Idioms]]

Current revision

Contents

1 Question

How to make a list type an instance of some type class in Haskell 98?

Haskell 98 does not support instances on particular composed types like
String
.


If I have a type class for conversion to a type
X
: 
class C a where
        toX   :: a -> X

I can define instances for 

instance C Int where toX  = ...
    instance C Double where toX  = ...
    instance C Tuple where toX  = ...
but not for Strings, given that they are a synonym for
[Char]
. Hence: 
instance C String where toX  = ...

results in: 

   Illegal instance declaration for `C String'
       (The instance type must be of form (T a b c)
        where T is not a synonym, and a,b,c are distinct type variables)
   In the instance declaration for `C String'

Is there some type class cleverness that can make this work in Haskell 98?

I'm aware of the approach taken by class
Show
in the prelude,

which adds a extra method to the class: 

class C a where
        toX     :: a -> X
        listToX :: [a] -> X
but I believe this says that whenever we can convert a to an
X
we can also convert
[a]
to an
X
, whereas I only want
[Char]
to be acceptable.

2 Answer

First you have to find out, what you really want to do.

2.1 Define a custom type

If you do not need list functions, but e.g. just want to use
String
values as identifiers,

then I advise to just define a custom type: 

newtype Identifier = Identifier String

and write the instances you need for that type.

2.2 Keep working with the list (or container) type

If you need list functions, then the wrapper approach means that you have to litter your code with calls to the wrapper constructor. In this case you should stick to the original composed type.

The trick in the Prelude for the
Show
class is, that
listToX
has a default implementation.

If this is not possible in your application then introduce a new class like 

class Element a where
        listToX :: [a] -> X

and define instances like 

instance Element Char where
        listToX = ...
 
    instance Element a => C [a] where
        toX = listToX

.

More generally, you can introduce a new class like 

class IsChar a where
        fromChar :: Char -> a
        toChar :: a -> Char
    instance IsChar Char where
        fromChar = id
        toChar = id
and then whenever you want to use the "
a
" type as a
Char
, you just convert it from or to
Char
with
fromChar
or
toChar
, respectively.

2.3 FlexibleInstances

If none of the above solution is appropriate for you, then you may choose the FlexibleInstances language extension that is available in GHC and Hugs. See http://www.haskell.org/ghc/docs/6.8-latest/html/users_guide/type-class-extensions.html#instance-rules 

{-# LANGUAGE FlexibleInstances #-}
    instance C [Char] where toX  = ...

3 Source

http://www.haskell.org/pipermail/haskell-cafe/2007-May/025742.html

Retrieved from "http://www.haskell.org/haskellwiki/List_instance"

Categories: FAQ | Idioms