List notation

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Revision as of 09:25, 19 May 2007

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:

  (x:) :: [a] -> [a]

  . E.g.

  iterate (' ':) []

  creates a list of blank strings with increasing size very efficiently.

See also:

• Haskell-Cafe thread: Comma in the front

Categories: Idioms | Syntax