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