# Difference between revisions of "List instance"

Benmachine (talk | contribs) (remove the need for TypeSynonymInstances in the example) |
(custom newtype is a nice solution in some cases) |
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If I have a type class for conversion to a type <hask>X</hask>: |
If I have a type class for conversion to a type <hask>X</hask>: |
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− | <haskell> |
+ | <haskell> |

class C a where |
class C a where |
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toX :: a -> X |
toX :: a -> X |
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</code> |
</code> |
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− | Is there some type class cleverness that can make this work in |
+ | Is there some type class cleverness that can make this work in Haskell 98? |

− | 98? I can create a new wrapper type for strings: |
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− | |||

− | and write an instance for that, but then I'll have to litter my code |
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− | with calls to this constructor. |
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<haskell> |
<haskell> |
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class C a where |
class C a where |
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== Answer == |
== Answer == |
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+ | First you have to find out, what you really want to do. |
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+ | === Define a custom type === |
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+ | If you do not need list functions, but e.g. just want to use <hask>String</hask> values as identifiers, |
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+ | then I advise to just define a custom type: |
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+ | |||

+ | and write the instances you need for that type. |
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+ | |||

+ | === Keep working with the list (or container) type === |
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+ | |||

+ | If you need list functions, |
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+ | then the wrapper approach means that you have to litter your code with calls to the wrapper constructor. |
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+ | In this case you should stick to the original composed type. |
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+ | The trick in the Prelude for the <hask>Show</hask> class is, |
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+ | |||

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<haskell> |
<haskell> |
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class Element a where |
class Element a where |
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listToX :: [a] -> X |
listToX :: [a] -> X |
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</haskell> |
</haskell> |
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and define instances like |
and define instances like |
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<haskell> |
<haskell> |
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instance Element Char where |
instance Element Char where |
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. |
. |
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− | ---- |
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<haskell> |
<haskell> |
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class IsChar a where |
class IsChar a where |
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toChar = id |
toChar = id |
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</haskell> |
</haskell> |
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+ | you just convert it from or to <hask>Char</hask> with <hask>fromChar</hask> or <hask>toChar</hask>, respectively. |
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+ | If none of the above solution is appropriate for you, |
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+ | then you may choose the <code>FlexibleInstances</code> language extension |
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+ | that is available in GHC and Hugs. |
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<haskell> |
<haskell> |

## Revision as of 13:38, 11 December 2010

## Contents

## 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.

## Answer

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

### 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.

### 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.

### 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 = ...
```

## Source

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