.FS File Format – Quotations

The quotation marks

<@ and @> must be separated from what is inside by a space only when that avoids reading them as part of an operator (<@ 1 + 2 @>). A list or array that begins with a quotation needs a space after the bracket ([ <@ 1 @> ]), because [< starts an attribute.

open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Quotations.Patterns

let q : Expr<int> = <@ 1 + 2 * 3 @>
let u : Expr = <@@ "text".Length @@>

let x = 10
let spliced = <@ %q + x @>                     // the value of x is captured; q is spliced in

let rec describe (e: Expr) =
    match e with
    | Value (v, _) -> sprintf "const %A" v
    | Call (None, mi, args) -> sprintf "%s(%s)" mi.Name (args |> List.map describe |> String.concat ", ")
    | Lambda (v, body) -> sprintf "fun %s -> %s" v.Name (describe body)
    | _ -> "?"

printfn "%s" (describe q.Raw)

let result = Linq.RuntimeHelpers.LeafExpressionConverter.EvaluateQuotation q   // evaluate

The Expr type and its shapes

An Expr is built by a quotation or by the static methods of the Expr class. The methods of FSharp.Quotations.Expr in the library interface are: AddressOf, AddressSet, Application, Applications, Call, CallWithWitnesses, Cast, Coerce, DefaultValue, Deserialize, FieldGet, FieldSet, ForIntegerRangeLoop, IfThenElse, Lambda, Let, LetRecursive, NewArray, NewDelegate, NewObject, NewRecord, NewStructTuple, NewTuple, NewUnionCase, PropertyGet, PropertySet, Quote, QuoteRaw, QuoteTyped, RegisterReflectedDefinitions, Sequential, Substitute, TryFinally, TryWith, TupleGet, TypeTest, UnionCaseTest, Value, ValueWithName, Var, VarSet, WhileLoop, WithValue, and the property and helper methods CustomAttributes, GetFreeVars, Type and TryGetReflectedDefinition.

The module Patterns has an active pattern for each of these shapes; DerivedPatterns adds the shapes that are built from them; ExprShape has one pattern that splits any expression into three shapes.

Quotations in the language

• A quotation may contain any expression the compiler can express as a tree; a few forms (some object-expression and byref uses) are not allowed. A quotation captures the value of a local variable from outside it, and a variable that is bound inside it stays a Var.
• Quotations are how query { … } is translated: the computation expression builder can request its body as a quotation through the builder’s Quote method (see computation expressions).
• F# 5 improved quotations (the documentation lists the change under “F# quotations improvements”).
• Quotation symbols are read by the lexer as the tokens <@, <@@, @>, @@> (and @>. and @>|} as shapes next to a dot or an anonymous-record end); the viewer colours them as directives.

Not described

Known, not yet covered

• Reference: the semantics of each Expr method (arguments and results), what a quotation of a particular construct looks like, ReflectedDefinition rules, the cases that cannot be quoted, serialisation (Deserialize), and the F# 5 improvements are only named. The lists of methods and patterns were taken from the names in quotations.fsi; the split between Patterns and DerivedPatterns was read from the line ranges of the two modules in that file.
• Reference: the example code was written for this page and not compiled.

See also

• Pattern matching and active patterns – the patterns used to walk a tree
• Lexical structure – the quotation tokens