# Difference between revisions of "List notation"

From HaskellWiki

(there are cool things possible with bare list syntax) |
(insert elements conditionally) |
||

Line 23: | Line 23: | ||

listB ++ | listB ++ | ||

[] | [] | ||

+ | </haskell> | ||

+ | * You can insert elements or sub-lists conditionally. | ||

+ | <haskell> | ||

+ | infixr 5 ?:, ?++ | ||

+ | |||

+ | (?:) :: (Bool, a) -> [a] -> [a] | ||

+ | (?:) (b, x) = if b then (x:) else id | ||

+ | |||

+ | (?++) :: (Bool, [a]) -> [a] -> [a] | ||

+ | (?++) (b, x) = if b then (x++) else id | ||

+ | |||

+ | list = | ||

+ | [2,3] ++ | ||

+ | (x==5, 5) ?: | ||

+ | (x==7, listA) ?++ | ||

+ | [] | ||

</haskell> | </haskell> | ||

* You can construct a singleton list with a [[operator section|section]] of the colon operator: <haskell>(:[]) :: a -> [a]</haskell>. | * You can construct a singleton list with a [[operator section|section]] of the colon operator: <haskell>(:[]) :: a -> [a]</haskell>. |

## Revision as of 09:25, 19 May 2007

We are used to the list notation `[0,1,2,3]`

.
However it is syntactic sugar for `(0:1:2:3:[])`

.
By using the syntactic sugar,
we often miss the benefits of the direct notation.

- A trailing colon is like a terminator.

```
0 :
1 :
2 :
3 :
[]
```

- Thus it is more theoretically sound and easier to edit.

- You can easily mix elements and lists into a list by appending the corresponding operator in each line:

```
[1,2,3] ++
4 :
listA ++
5 :
listB ++
[]
```

- You can insert elements or sub-lists conditionally.

```
infixr 5 ?:, ?++
(?:) :: (Bool, a) -> [a] -> [a]
(?:) (b, x) = if b then (x:) else id
(?++) :: (Bool, [a]) -> [a] -> [a]
(?++) (b, x) = if b then (x++) else id
list =
[2,3] ++
(x==5, 5) ?:
(x==7, listA) ?++
[]
```

- You can construct a singleton list with a section of the colon operator: .
(:[]) :: a -> [a]

- You can prepend an element to a list: . E.g.
(x:) :: [a] -> [a]

creates a list of blank strings with increasing size very efficiently.iterate (' ':) []

See also: