Version 10 (modified by guest, 3 years ago) (diff)

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This wiki page documens the design and implementation of the GHC extension for overloading Haskell's list notation.

## Current Implementation

Let us briefly recap the notation for constructing lists. In Haskell, the list notation can be be used in the following seven ways:

```[]          -- Empty list
[x]         -- x : []
[x,y,z]     -- x : y : z : []
[x .. ]     -- enumFrom x
[x,y ..]    -- enumFromThen x y
[x .. y]    -- enumFromTo x y
[x,y .. z]  -- enumFromThenTo x y z
```

When the OverloadedLists extension is turned on, the aforementioned seven notations are desugared as follows:

```[]          -- fromListN 0 []
[x]         -- fromListN 1 (x : [])
[x,y,z]     -- fromListN 3 (x : y : z : [])
[x .. ]     -- fromList (enumFrom x)
[x,y ..]    -- fromList (enumFromThen x y)
[x .. y]    -- fromList (enumFromTo x y)
[x,y .. z]  -- fromList (enumFromThenTo x y z)
```

This extension allows programmers to use the list notation for construction of structures like: Set, Map, IntMap, Vector, Text and Array. The following code listing gives a few examples:

```['0' .. '9']             :: Set Char
[1 .. 10]                :: Vector Int
[("default",0), (k1,v1)] :: Map String Int
['a' .. 'z']             :: Text
```

List patterns are also overloaded. When the OverloadedLists extension is turned on, the definitions

```f [] = ...
g [x,y,z] = ...
```

will be treated as

```f (toList -> []) = ...
g (toList -> [x,y,z]) = ...
```

GHC, during the typechecking and desugaring phases, uses whatever is in scope with the names of fromList, toList and fromListN (i.e., fromList, toList and fromListN are rebindable).

That said, the GHC.Exts module exports the IsList class that can be used to overload fromListN and fromListN for different structures. The type class is defined as follows:

```class IsList l where
type Item l
fromList  :: [Item l] -> l
toList    :: l -> [Item l]

fromListN :: Int -> [Item l] -> l
fromListN _ = fromList
```

The IsList class and its methods are intended to be used in conjunction with the OverloadedLists extension. The Item type function returns the type of items of the structure l. The fromList function constructs the structure l from the given list of Item l. The fromListN function takes the input list's length as a hint. Its behaviour should be equivalent to fromList. The hint can be used for more efficient construction of the structure l compared to fromList. If the given hint is not equal to the input list's length the behaviour of fromListN is not specified.

The instances of the IsList class should satisfy the following property:

```fromList . toList = id
```

In the following, we give several example instances of the IsList type class:

```instance IsList [a] where
type Item [a] = a
fromList = id
toList   = id

instance (Ord a) => IsList (Set a) where
type Item (Set a) = a
fromList = Set.fromList
toList   = Set.toList

instance (Ord k) => IsList (Map k v) where
type Item (Map k v) = (k,v)
fromList = Map.fromList
toList   = Map.toList

instance IsList (IntMap v) where
type Item (IntMap v) = (Int,v)
fromList = IntMap.fromList
toList   = IntMap.toList

instance IsList Text where
type Item Text = Char
fromList = Text.pack
toList   = Text.unpack

instance IsList (Vector a) where
type Item (Vector a) = a
fromList  = Vector.fromList
toList    = Vector.toList
fromListN = Vector.fromListN

```

## Defaulting

Currently, the IsList class is not accompanied with defaulting rules. Although feasible, not much thought has gone into how to specify the meaning of the default declarations like: default ([a])

## Literal Lists

The current implementation of the OverloadedLists extension can be improved by handling the lists that are only populated with literals in a special way. More specifically, the compiler could allocate such lists statically using a compact representation and allow IsList instances to take advantage of the compact representation. Equipped with this capability the OverloadedLists extension will be in a good position to subsume the OverloadedStrings extension (currently, as a special case, string literals benefit from statically allocated compact representation).

Somewhat related discussions:

```http://hackage.haskell.org/trac/ghc/ticket/5218
http://www.serpentine.com/blog/2012/09/12/the-case-of-the-mysterious-explosion-in-space/
http://www.mail-archive.com/[email protected]
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