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on 2/26/2014 6:07 AM

Anyone who has written non-trivial applications that need to access a SQL database can attest it: being able to compose SQL queries, insert parts of query that depend on some runtime criterion, is tremendously helpful. Not having this capability often means having to copy and paste queries just to be able to add a "where" or a "join" clause, and I know few better ways to end up with buggy, hard-to-maintain code.

This article discusses the auto-quoting mechanism of F# 3.x query expressions and how it can be leveraged to implement not-too-clunky composition.

## PowerPack.Linq

In F# 2.0, the standard / usual way to access SQL databases was PowerPack.Linq, which featured a query function capable of translating a quotation containing sequence operations into a LINQ query. Therefore, composing sub-queries basically meant composing sequence operations, and it was not too difficult: quotation splicing did the trick.

Let's take the classic example of a web forum, with a Message table containing a field UserId that keys into a User table through its Id field. Let's write the following function:

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val GetLatestMessages : num: int -> username: string option -> Message[]

If username is None, we return the num latest messages posted by anyone; else, only those posted by the given user. So depending on username, we will need two different SQL queries: one with a join into the users table, and one without.

With PowerPack.Linq, it will look like this:

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let GetLatestMessages num usernameOpt =
use db = (* retrieve LINQ-to-SQL context *)
let baseQuery =
<@ db.Messages |> Seq.sortBy (fun m -> m.PostedDate) @>
let filteredQuery =
| None -> baseQuery
<@ seq { for m in %baseQuery do
for u in db.Users do
yield m } @>
query <@ %filteredQuery |> Seq.take num @>
|> Array.ofSeq

Note the use of the percent operator. It performs quotation splicing, which means that its argument, itself a quotation, is put as-is into the quotation being defined. In the end, the quotation compiled by query depends on the value of usernameOpt. If it is None, then the final quotation is:

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<@ db.Messages
|> Seq.sortBy (fun m -> m.PostedDate)
|> Seq.take num @>

If it is Some username, then the final quotation is:

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<@ seq { for m in (db.Messages |> Seq.sortBy (fun m -> m.PostedDate)) do
for u in db.Users do
yield m }
|> Seq.take num @>

The function query is then able to take whichever of these quotations, compile it into a valid SQL query, and run it. We do have the functionality we need, with the totality of the query being performed by the database.

## Query Expressions: The Problem

F# 3.0 introduced query expressions, with the intent of creating a syntax similar to C#'s own LINQ syntax, itself very close to plain SQL. Using query expressions, the two above variants would be written as follows:

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query { for m in db.Messages do
sortBy m.PostedDate
select m
take num }

query { for m in db.Messages do
sortBy m.PostedDate
join u in db.Users on (m.UserId = u.Id)
select m
take num }

But how can we choose between these two queries without having to duplicate the common parts, just like we did with PowerPack.Linq? As a first attempt, we can try to use the result of the previous fragment in the for part of the next one.

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let GetLatestMessages num usernameOpt =
use db = (* retrieve LINQ-to-SQL context *)
let baseQuery =
query { for m in db.Messages do
sortBy m.PostedDate
select m }
let filteredQuery =
| None -> baseQuery
query { for m in baseQuery do
join u in db.Users on (m.UserId = u.Id)
select m }
query { for m in filteredQuery do
take num }
|> Array.ofSeq

However, at runtime, we get this cryptic exception (using a LINQ to Entities context; the message will probably be different if you use LINQ to SQL):

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This method supports the LINQ to Entities infrastructure
and is not intended to be used directly from your code.

What is going on here? Well, the problem here is that the query builder actually performs the translation to LINQ. So baseQuery is actually an IQueryable<Message> representing the output of the query. It is therefore too late to combine into a more complex query!

## Query Expressions: The Solution

In order to progress further, we need to understand how query expressions work internally. Like any F# computation expression, a query expression is mostly syntactic sugar on top of method calls on the builder object used to create the expression, in this case the query object. For example, baseQuery above corresponds to the following expression:

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  query.Select(
query.SortBy(
query.Source(db.Messages),
fun m -> m.PostedDate),
fun m -> m)

(I am actually omitting a couple extra method calls that would make this less readable, but the principle stays the same.)

But actually... Not quite. If this was all there was to it, then it would be trivially composable, just like normal seq { ... } expressions are composable. But in order to be able to compile such an expression to LINQ, it needs to be able to inspect it; and to do so, without telling you, it uses... (drumrolls...) quotations.

In fact, the query expression syntax does more than put sugar on top of some method calls; when some conditions are met (ie. when the builder object has certain specific methods), it also implicitly puts the whole body in a quotation, and calls the Run method on it. The translation of baseQuery is therefore closer to this:

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query.Run <@
query.Select(
query.SortBy(
query.Source(db.Messages),
fun m -> m.PostedDate),
fun m -> m)
@>

query.Run : Expr<Linq.QuerySource<'T, _>> -> IQueryable<'T> is responsible for taking this quotation and turning it into the final LINQ expression that maps directly to SQL.

Now if only we could have direct access to the quotation, we would be golden. Well, it turns out we can do exactly that. We are going to play around with quoting and unquoting pieces of queries, which is something I've first seen done by Tomas Petricek here. In fact, this SO response is the original inspiration for this whole post.

All we have to do here is to create a subclass of Linq.QueryBuilder (the class of query) that implements Run differently.

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type PartialQueryBuilder() =
inherit Linq.QueryBuilder()

member this.Run(e: Expr<Linq.QuerySource<'T, IQueryable>>) = e

let pquery = PartialQueryBuilder()

We now have a query expression constructor, pquery, that we can invoke exactly like query, except that it returns the unevaluated query quotation.

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let baseQuery =
pquery{ for m in db.Messages do
sortBy m.PostedDate
select m }

// val baseQuery : Expr<Linq.QuerySource<'T, _>>

What we want to do next is to be able to pass this unevaluated query to for in another query. As I've shown above, the sequence given to for is passed by the desugaring to a method query.Source : IQueryable<'T> -> Linq.QuerySource<'T, _>. But here, pquery directly gives us a Linq.QuerySource<'T, _>. No problem, we can use a trick similar to the above Run trick, except using an extension method on query itself:

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type Linq.QueryBuilder with
[<ReflectedDefinition>]
member this.Source(qs: Linq.QuerySource<'T, _>) = qs

Okay, now all we have to do is pass our partial query to for. But wait, isn't it an Expr<Linq.QuerySource<'T, _>>, ie. a quoted version of what the Source we just wrote expects? Well, remember when I said that the body of query expressions gets wrapped in a quotation? This is not just a figure, it really is exactly what happens. So you can even use the % splicing operator in query expressions! For our example, the following code is valid:

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query { for m in %baseQuery do
join u in db.Users on (m.UserId = u.Id)
select m }

And there we go! We just wrote a partial query and composed it into a final query, and readability-wise all it takes is an extra p in the former and a % in the latter. Not bad!

Here is the whole code for reference:

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// Utility code, write it once

type PartialQueryBuilder() =
inherit Linq.QueryBuilder()

member this.Run(e: Expr<Linq.QuerySource<'T, IQueryable>>) = e

let pquery = PartialQueryBuilder()

type Linq.QueryBuilder with
[<ReflectedDefinition>]
member this.Source(qs: Linq.QuerySource<'T, _>) = qs

// Example usage

use db = (* retrieve LINQ-to-SQL context *)
let baseQuery =
pquery{ for m in ctx.Messages do
sortBy m.PostedDate
select m }
let filteredQuery =
| None -> baseQuery
|> Array.ofSeq
Note how we can just as easily compose a partial query into another partial query. And finally, if you need to directly run a partial query (as opposed to splicing it into a normal query), you can just pass it to the standard query.Run.