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The first line after the match is a pattern-matching rule that matches the input primes against the pattern h :: t. If primes is a nonempty list, then the match will be successful, and the first printfn will be executed with h bound to the head of the list and t to its tail. The second line considers the case where primes is an empty list. Note that the :: and [] symbols can be used both to build up lists in expressions and to decompose them in pattern matching. The F# library also includes a module List that contains some useful functions related to programming with lists. You ll be seeing many of these functions in the next section and throughout this book. Table 3-9 shows some of these. F# lists are not appropriate for all circumstances; for example, very large data structures should probably be represented using arrays or other data structures or even managed by an external tool such as a relational database. We discuss a number of immutable data structures in the Some Common Immutable Data Structures sidebar.

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1 2 ... 342,023

123 456 ... 987

Returns the length of the list. Returns the first element of a nonempty list. Returns all the elements of a nonempty list except the first. Returns a new list of length given by the first parameter and elements generated by the second function parameter. Returns a new list containing the elements of the first list followed by the elements of the second list. Returns a new list containing only those elements of the original list where the function returns true. Returns a new list where the function has been applied to each element of the list. Executes the given function for each element of the list. Returns two new lists containing the first and second elements of the pairs in the input list. Returns a new list containing the elements of the two input lists combined pairwise. The input lists must be the same length; otherwise, an exception is raised. Converts the list to an array. Converts the array to a list.

Note I do not mean to imply that rows have any sort of physical ordering on disk in this example. There really

is not a first row, second row, or last row in a table. There is just a set of tables. We are assuming here that row 1 really means the first row we happened to read and row 2 is the second row we happened to read and so on.

: ('a -> 'b )

At some point while we are in the middle of the query, a transaction moves $400.00 from account 123 to account 987. This transaction does the two updates but does not commit. The table now looks as shown in Table 7-3. Table 7-3. ACCOUNTS Table During Modifications

1 2 ... 342,023

Here are examples of how to use some of the functions from Table 3-9. The last two examples use function values, which we cover in more detail in Introducing Function Values later in this chapter. > List.hd [5; 4; 3];; val it : int = 5 > List.tl [5; 4; 3];; val it : int list = [ 4; 3 ] > List.map (fun x -> x*x) [1; 2; 3];; val it : int list = [ 1; 4; 9 ] > List.filter (fun x -> x % 3 = 0) [2; 3; 5; 7; 9];; val it : int list = [ 3; 9 ]

123 456 ... 987

So, two of those rows are locked If anyone tried to update them, that user would be blocked So far, the behavior we are seeing is more or less consistent across all databases The difference will be in what happens when the query gets to the locked data When the query we are executing gets to the block containing the locked row (row 342,023) at the bottom of the table, it will notice that the data in it has changed since the time at which it started execution To provide a consistent (correct) answer, Oracle will at this point create a copy of the block containing this row as it existed when the query began That is, it will read a value of $10000, the value that existed at the time the query began.