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? |
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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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| + | Example: |
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| + | <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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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. |
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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