# Singleton list confusion

### From HaskellWiki

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Why do Haskell newcomers frequently believe, that list variables must be enclosed in brackets? |
Why do Haskell newcomers frequently believe, that list variables must be enclosed in brackets? |
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People sometimes write argument patterns like <hask>[x]</hask>, hoping that <hask>x</hask> will assume all values of a list successively. |
People sometimes write argument patterns like <hask>[x]</hask>, hoping that <hask>x</hask> will assume all values of a list successively. |
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− | Maybe they expect some kind of [[list comprehension]]. |
+ | Example: |

+ | <haskell> |
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+ | case xs of |
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+ | [x] -> [f x] |
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+ | </haskell> |
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+ | although they meant |
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+ | <haskell> |
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+ | [f x | x <- xs] |
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+ | </haskell> |
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+ | or just <hask>map f xs</hask>. |
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+ | It seems like they expect some kind of [[list comprehension]]. |
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However, if there wouldn't be a special [[list notation]] and if there wouldn't be the special syntax for the list type constructor (thus we must write <hask>[a]</hask> instead of <hask>List a</hask>), |
However, if there wouldn't be a special [[list notation]] and if there wouldn't be the special syntax for the list type constructor (thus we must write <hask>[a]</hask> instead of <hask>List a</hask>), |
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then there would be certainly less confusion. |
then there would be certainly less confusion. |

## Latest revision as of 07:16, 11 April 2008

Why do Haskell newcomers frequently believe, that list variables must be enclosed in brackets?

People sometimes write argument patterns like[x]

x

Example:

case xs of [x] -> [f x]

although they meant

[f x | x <- xs]

map f xs

It seems like they expect some kind of list comprehension.

However, if there wouldn't be a special list notation and if there wouldn't be the special syntax for the list type constructor (thus we must write[a]

List a

then there would be certainly less confusion.

See for example Haskell-Cafe about testing for same characters in lists of strings, Pattern match failure