Query

With LINQ we can use advanced query expressions in F# code. These queries, which are specified with the query keyword, are similar to SQL statements on native code.

In F#, we begin query expressions with the query keyword, and then the "for" keyword. We can use select to get values as a result, or other functions like count or lastOrDefault.

Example

We test 4 different query expressions in this example. Some of the queries return a single result value, and others return a sequence we evaluate with Seq.iter.

Part 1 With this query, we count all the elements in the array of numbers. Since there are 4 elements, the result is 4.

Part 2 It is possible to use sorting in query expressions. We use the sortBy clause to order the numbers from low to high.

Part 3 With "where" we filter elements with a query. Here we keep numbers only greater than or equal to 50, which leaves 55.

Part 4 With lastOrDefault, we either keep the last result if one exists, or the default value—which for int is 0.

let numbers = [45; 5; 55; 10]

// Part 1: use query expression with count.
let query1 = query { for number in numbers do
                     select number
                     count }
printfn $"QUERY1: {query1}"

// Part 2: use query with sortBy and display with Seq.iter.
let query2 = query { for number in numbers do
                     sortBy number
                     select number }
query2
|> Seq.iter (fun n -> printfn $"QUERY2: {n}")

// Part 3: use query with where.
let query3 = query { for number in numbers do
                     where (number >= 50)
                     select number }
for n in query3 do
    printfn $"QUERY3: {n}"

// Part 4: use query with where and lastOrDefault.
let query4 = query { for number in numbers do
                     where (number = 3000)
                     lastOrDefault }
printfn $"QUERY4: {query4}"QUERY1: 4
QUERY2: 5
QUERY2: 10
QUERY2: 45
QUERY2: 55
QUERY3: 55
QUERY4: 0

Query results

A query returns an IEnumerable collection. We can evaluate the contents of the IEnumerable in F# with Seq.iter or with a for-do loop.

And The results are discovered lazily. So the queries are only executed when the Seq.iter or for-do are run.

While the query keyword block complicates syntax, queries in F# are still powerful and concise. They are probably the easiest way to perform complex sorts and filtering operations.