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Validity of program identifiers in only the declaring scope

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Local scope exists in many forms for most programming languages.

Consider the following implementation of the factorial function in Language:Java:

  // An imperative definition of the factorial function
  public static int factorial(int n) {
    // n and result use method scope
    int result = 1;
    // i uses the for loop as scope
    for (int i=n; i>1; i--) 
      result = result * i;
    return result;
  }

Several kinds of local scope are present. The parameter of the method, n, has local scope with the entire method corresponding to the scope. The local variable result has that same local scope, but it is declared differently. The index variable i is declared in the for loop and its scope is indeed limited to the for loop.

Consider the following Language:Haskell function for splitting a list into two halves (+/-1 in the case of a list of odd length):

-- Split a list into halves
split :: [Int] -> ([Int],[Int])
split xs = (take len xs, drop len xs)
  where
    len = length xs `div` 2

Local scope is present again in several ways. First, the argument xs of the split function is locally bound in the given equation; specifically it can be used on the right-hand side of the equation. Further, the split function carries a "where" block which is used to define a local binding len. Again, len can only be used within the scope of split. It is also worth noting that len makes good use of the argument xs which has been bound by the hosting function equation for split.

The use of local definitions like len greatly improves clarity of programming. If we were to avoid the local binding, then the split function would need to duplicate some code as follows:

-- Splitting without using local scope
split' :: [Int] -> ([Int],[Int])
split' xs =
  ( take (length xs `div` 2) xs, 
    drop (length xs `div` 2) xs )

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