# List notation

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Revision as of 17:23, 10 January 2014 by Lemming (talk | contribs) (one more example: Alternating list)

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 (' ':) []

- You can adapt this style to other list-like data structures, e.g. a list of elements with alternating element types. See e.g. event-list.

```
data Alternating a b = Alternating a [(b,a)]
infixr 5 /., ./
(/.) :: a -> [(b,a)] -> Alternating a b
(/.) = Alternating
(./) :: b -> Alternating a b -> [(b,a)]
b ./ Alternating a bas = (b,a) : bas
example :: Alternating Bool Int
example = True /. 0 ./ False /. 1 ./ True /. []
```