A quotation mechanism lets you embed code in your code and have the compiler transform that code from the source you provide into a data structure that represents it. For example, the following gives you a data structure representing the F# expression
1+2
:> <@ 1+2 @>;;
val it : Quotations.Expr<int> =
Call (None, Int32 op_Addition[Int32,Int32,Int32](Int32, Int32),
[Value (1), Value (2)])
{CustomAttributes = [NewTuple (Value ("DebugRange"),
NewTuple (Value ("stdin"), Value (3), Value (3), Value (3), Value (6)))];
Raw = ...;
Type = System.Int32;}
You can then hack on this data structure in order to apply transformations to your code, such as translating it from F# to Javascript in order to run it client side on almost any browser.
The F# quotation mechanism is extremely limited in functionality compared to the quotation mechanisms of languages like OCaml and Lisp. Moreover, although the .NET Framework and F# compiler provide everything required to compile and execute quoted code at full speed, the evaluation mechanism for quoted code is orders of magnitude slower than real F# code which, again, renders it virtually useless. Consequently, I am not familiar with any real applications of it beyond Websharper.
For example, you can only quote certain kinds of expressions in F# and not other code such as type definitions:
> <@ type t = Int of int @>;;
<@ type t = Int of int @>;;
---^^^^
C:\Users\Jon\AppData\Local\Temp\stdin(4,4): error FS0010: Unexpected keyword 'type' in quotation literal
Most quotation mechanisms let you quote any valid code at all. For example, OCaml's quotation mechanism can quote the type definition that F# just barfed on:
$ ledit ocaml dynlink.cma camlp4oof.cma
Objective Caml version 3.12.0
Camlp4 Parsing version 3.12.0
# open Camlp4.PreCast;;
# let _loc = Loc.ghost;;
val _loc : Camlp4.PreCast.Loc.t = <abstr>
# <:expr< 1+2 >>;;
- : Camlp4.PreCast.Ast.expr =
Camlp4.PreCast.Ast.ExApp (<abstr>,
Camlp4.PreCast.Ast.ExApp (<abstr>,
Camlp4.PreCast.Ast.ExId (<abstr>, Camlp4.PreCast.Ast.IdLid (<abstr>, "+")),
Camlp4.PreCast.Ast.ExInt (<abstr>, "1")),
Camlp4.PreCast.Ast.ExInt (<abstr>, "2"))
# <:str_item< type t = Int of int >>;;
- : Camlp4.PreCast.Ast.str_item =
Camlp4.PreCast.Ast.StSem (<abstr>,
Camlp4.PreCast.Ast.StTyp (<abstr>,
Camlp4.PreCast.Ast.TyDcl (<abstr>, "t", [],
Camlp4.PreCast.Ast.TySum (<abstr>,
Camlp4.PreCast.Ast.TyOf (<abstr>,
Camlp4.PreCast.Ast.TyId (<abstr>,
Camlp4.PreCast.Ast.IdUid (<abstr>, "Int")),
Camlp4.PreCast.Ast.TyId (<abstr>,
Camlp4.PreCast.Ast.IdLid (<abstr>, "int")))),
[])),
Camlp4.PreCast.Ast.StNil <abstr>)
Here is an example in Common Lisp:
$ sbcl
This is SBCL 1.0.29.11.debian, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.
SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses. See the CREDITS and COPYING files in the
distribution for more information.
* '(+ 1 2)
(+ 1 2)
Metaprogramming is one application where pattern matching can be extremely useful but pattern matching is a general-purpose language feature. You may appreciate the article from the Benefits of OCaml about a minimal interpreter. In particular, note how easy pattern matching makes it to act upon each of the different kinds of expression:
> let rec eval vars = function
| EApply(func, arg) ->
match eval vars func, eval vars arg with
| VClosure(var, vars, body), arg -> eval ((var, arg) :: vars) body
| _ -> invalid_arg "Attempt to apply a non-function value"
| EAdd(e1, e2) -> VInt (int(eval vars e1) + int(eval vars e2))
| EMul(e1, e2) -> VInt (int(eval vars e1) * int(eval vars e2))
| EEqual(e1, e2) -> VBool (eval vars e1 = eval vars e2)
| EIf(p, t, f) -> eval vars (if bool (eval vars p) then t else f)
| EInt i -> VInt i
| ELetRec(var, arg, body, rest) ->
let rec vars = (var, VClosure(arg, vars, body)) :: vars in
eval vars rest
| EVar s -> List.assoc s vars;;
val eval : (string * value) list -> expr -> value = <fun>
That OCaml article was used as the basis of the F#.NET Journal article "Language-oriented programming: The Term-level Interpreter" (31st December 2007).
You can write compilers in F#. In fact, F# is derived from a family of languages that were specifically designed for metaprogramming, the so-called MetaLanguages (ML) family.
The article "Run-time code generation using System.Reflection.Emit" (31st August 2008) from the F#.NET Journal described the design and implementation of a simple compiler for a minimal language called Brainf*ck. You can extend this to implement more sophisticated languages like Scheme. Indeed, the F# compiler is mostly written in F# itself.
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