This is a guest post by Max Hirschhorn,who is currently an intern at MongoDB. About the F# programming language F# is a multi-paradigm language built on the .NET framework. It isfunctional-first and prefers immutability, but also supportso
This is a guest post by Max Hirschhorn, who is currently an intern at MongoDB.
About the F# programming language
F# is a multi-paradigm language built on the .NET framework. It is functional-first and prefers immutability, but also supports object-oriented and imperative programming styles.
Also, F# is a statically-typed language with a type inference system. It has a syntax similar to Ocaml, and draws upon ideas from other functional programming languages such as Erlang and Haskell.
Using the existing .NET driver
The existing .NET driver is compatible with F#, but is not necessarily written in a way that is idiomatic to use from F#.
Part of the reason behind this is that everything in F# is explicit. For example, consider the following example interface and implementing class.
[] type I = abstract Foo : unit -> string type C() = interface I with member __.Foo () = "bar" // example usage let c = C() (c :> I).Foo()
So in order to use any of the interface members, the class must be
upcasted using
the :>
operator. Note that this cast is still checked at compile-time.
In a similar vein, C# supports implicit
operators,
which the BSON library uses for converting between a primitive value
and its BsonValue
equivalent, e.g.
new BsonDocument { { "price", 1.99 }, { "$or", new BsonDocument { { "qty", new BsonDocument { { "$lt", 20 } } }, { "sale", true } } } };
whereas F# does not. This requires the developer to explicitly
construct the appropriate type of BsonValue
, e.g.
BsonDocument([ BsonElement("price", BsonDouble(1.99)) BsonElement("$or", BsonArray([ BsonDocument("qty", BsonDocument("$lt", BsonInt32(20))) BsonDocument("sale", BsonBoolean(true)) ])) ])
with the query builder, we can hide the construction of BsonDocument
instances, e.g.
Query.And([ Query.EQ("price", BsonDouble(1.99)) Query.OR([ Query.LT("qty", BsonInt32(20)) Query.EQ("sale", BsonBoolean(true)) ]) ])
It is worth noting that the need to construct the BsonValue
instances
is completely avoided when using a typed QueryBuilder
.
type Item = { Price : float Quantity : int Sale : bool } let query = QueryBuilder() query.And([ query.EQ((fun item -> item.Price), 1.99) query.Or([ query.LT((fun item -> item.Quantity), 20) query.EQ((fun item -> item.Sale), true) ]) ])
What we are looking for is a solution that matches the brevity of F# code, offers type-safety if desired, and is easy to use from the language.
New features
The main focus of this project is to make writing queries against MongoDB as natural from the F# language as possible.
bson
quotations
We strive to make writing predicates as natural as possible by reusing as many of the existing operators as possible.
A taste
Consider the following query
{ price: 1.99, $or: [ { qty: { $lt: 20 } }, { sale: true } ] }
we could express this with a code quotation
bson x?price = 1.99 && (x?qty
or with type safety
bson x.Price = 1.99 && (x.Quantity
Breaking it down
The quotations are not actually executed, but instead are presented
as an abstract syntax tree (AST), from which an equivalent
BsonDocument
instance is constructed.
The ?
operator
The ?
operator is defined to allow for an unchecked comparison. The
F# language supports the ability to do a dynamic lookup (get) and
assignment (set) via the ?
and ? operators respectively, but does
not actually provide a implementation.
So, the F# driver defines the ?
operator as the value associated with
a field in a document casted to a fresh generic type.
// type signature: BsonDocument -> string -> 'a let (?) (doc : BsonDocument) (field : string) = unbox doc.[field]
and similarly defines the ? operator as the coerced assignment of a
generically typed value to the associated field in the document.
// type signature: BsonDocument -> string -> 'a -> unit let (? ignore
Queries
Unchecked expressions have the type signature
Expr<bsondocument> bool></bsondocument>
.
// $mod bson x?qty % 4 = 0 @>
Checked expressions have the type signature Expr bool>
.
// $mod bson x.Quantity % 4 = 0 @>
Updates
Unchecked expressions have the type signature
Expr<bsondocument> unit list></bsondocument>
. The reason for the list
in the
return type is to perform multiple update operations.
// $set bson [ x?qty // $inc bson [ x?qty
Mmm… sugar
A keen observer would notice that (+) 1
is not an int
, but actually
a function int -> int
. We are abusing the fact that type safety is
not enforced here by assigning the quantity field of the document to a
lambda expression, that takes a single parameter of the current value.
Note that
// $inc bson [ x?qty
is also valid.
Checked expressions either have the type signature
Expr unit list>
or Expr 'DocType>
,
depending on whether the document type has mutable fields (only matters
for record types).
// $set bson [ x.Quantity // $inc bson [ x.Quantity
mongo
expressions
Uses the monadic structure (computation expression) to define a pipeline of operations that are executed on each document in the collection.
Queries
let collection : IMongoCollection = ... mongo { for x in collection do where (x?price = 1.99 && (x?qty <p>or with a typed collection</p> <pre class="brush:php;toolbar:false"> let collection : IMongoCollection = ... mongo { for x in collection do where (x.price = 1.99 && (x.qty <h5 id="Updates">Updates</h5> <pre class="brush:php;toolbar:false"> let collection : IMongoCollection = ... mongo { for x in collection do update set x?price 0.99 inc x?qty 1 }
or with a typed collection
let collection : IMongoCollection = ... mongo { for x in collection do update set x.Price 0.99 inc x.Quantity 1 }
Serialization of F# data types
Now supports
- record types
- option types
- discriminated unions
Conclusion
Resources
The source code is available at GitHub. We absolutely encourage you to experiment with it and provide us feedback on the API, design, and implementation. Bug reports and suggestions for improvements are welcomed, as are pull requests.
Disclaimer. The API and implementation are currently subject to change at any time. You must not use this driver in production, as it is still under development and is in no way supported by MongoDB, Inc.
Acknowledgments
Many thanks to the guidance from the F# community on Twitter, and my mentors: Sridhar Nanjundeswaran, Craig Wilson, and Robert Stam. Also, a special thanks to Stacy Ferranti and Ian Whalen for overseeing the internship program.
原文地址:Enhancing the F# developer experience with MongoDB, 感谢原作者分享。

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