# Difference between revisions of "Singleton list confusion"

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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? | ||

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. | ||

− | + | Example: | |

+ | <haskell> | ||

+ | case xs of | ||

+ | [x] -> [f x] | ||

+ | </haskell> | ||

+ | although they meant | ||

+ | <haskell> | ||

+ | [f x | x <- xs] | ||

+ | </haskell> | ||

+ | or just <hask>map f xs</hask>. | ||

+ | 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 <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>), | ||

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]`

, hoping that `x`

will assume all values of a list successively.
Example:

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

although they meant

```
[f x | x <- xs]
```

or just `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]`

instead of `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