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(Informatik 2)
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| Context Patterns |  |
| Introductory Example |
| If you | try to describe how to program a function
initlast which splits a list into its initial
part and its last element you get something like
Take everything up to but not including a one--element list as initial part and the element of the list as last element. |
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| However, | using the standard patterns, you cannot express
this. Instead, the recursive search must be programmed
explicitly:
initlast :: [a] -> (a,[a])
initlast [] = error "Empty list"
initlast [x] = ([],x)
initlast (x:xs) = let (ys,l)=initlast xs
in (x:ys,l)
Ugly, isn't it?!
|
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| Context patterns | allow a much shorter and more concise program:
initlast :: [a] -> (a,[a]) initlast [] = error "Empty list" initlast (c [x]) = ((c []), x)The context pattern (c [x]) has the variable c in the position of a function and contains the context of [x]: Everything of the list but [x]. |
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| Applying | this initlast to a list
[a1,...,an-1,an]
results in
|
Context Patterns
Markus Mohnen Last modified: Tue Sep 2 09:35:54 MET DST 1997
