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CHAPTER 9 INTRODUCING LANGUAGE-ORIENTED PROGRAMMING
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type CheeseClub = { [<Column(0)>] Name : string [<Column(2)>] FavouriteCheese : string [<Column(1)>] LastAttendance : System.DateTime } You can now instantiate the SchemaReader type and use it to read the data from the file into this typed format: > let reader = new SchemaReader<CheeseClub>();; val reader : SchemaReader<CheeseClub> > fsi.AddPrinter(fun (c:System.DateTime) -> c.ToString());; val it : unit = () > System.IO.File.WriteAllLines("data.txt", [| "Steve, 12 March 2010, Cheddar"; "Sally, 18 Feb 2010, Brie"; |]);; val it : unit = () > reader.ReadFile("data.txt");; val it : seq<CheeseClub> = seq [{Name = "Steve"; FavouriteCheese = "Cheddar"; LastAttendance = 12/03/2010 00:00:00;}; {Name = "Sally"; FavouriteCheese = "Brie"; LastAttendance = 18/02/2010 00:00:00;}] There is something somewhat magical about this; you ve built a layer that automatically does the impedance matching between the untyped world of a text-file format into the typed world of F# programming. Amazingly, the SchemaReader type itself is only about 50 lines of code. The comments in Listing 9-10 show the basic steps being performed. The essential features of this technique are as follows: 1. The schema information is passed to the SchemaReader as a type variable. The SchemaReader then uses the typeof operator to extract a System.Type representation of the schema type. The information needed to drive the transformation process comes from custom attributes. Extra information can also be supplied to the constructor of the SchemaReader type if necessary. The let bindings of the SchemaReader type are effectively a form of precomputation (they can also be seen as a form of compilation). They precompute as much information as possible given the schema. For example, the section analyzes the fields of the schema type and computes functions for creating objects of the field types. It also computes the permutation from the text file columns to the record fields. The data objects are ultimately constructed using reflection functions, in this case a function computed by Microsoft.FSharp.Reflection.Value. GetRecordConstructor or primitive values parsed using System.Int32.Parse and similar functions. This and other functions for creating F# objects dynamically
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CHAPTER 9 INTRODUCING LANGUAGE-ORIENTED PROGRAMMING
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are in the Microsoft.FSharp.Reflection library. Other functions for creating other .NET objects dynamically are in the System.Reflection library. 5. The member bindings of SchemaReader interpret the residue of the precomputation stage, in this case using the information and computed functions to process the results of splitting the text of a line.
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This technique has many potential applications and has been used for CSV file reading, building F#-centric serializers/deserializers, and building generic strongly typed database schema access.
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Using the F# Dynamic Reflection Operators
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F# lets you define two special operators, ( ) and ( <-), to perform dynamic lookups of objects. These are conceptually very simple operators, but they add interesting new opportunities for interoperability between dynamic data and static data in F# programming. These operators implicitly translate their second argument to a string, if it s a simple identifier. That is, a use of these operators is translated as follows: expr nm expr1 nm <- expr2 ( ) expr "nm" ( <-) expr1 "nm" expr2
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This means the operators can be used to simulate a dynamic lookup of a property or a method on an object. This dynamic lookup can use any dynamic/reflective technique available to you. One such technique is to use .NET reflection to look up and/or set the properties of an object: open System.Reflection let ( ) (obj:obj) (nm:string) : 'T = obj.GetType().InvokeMember(nm, BindingFlags.GetProperty, null, obj, [| |]) |> unbox<'T> let ( <-) (obj:obj) (nm:string) (v:obj) : unit = obj.GetType().InvokeMember(nm, BindingFlags.SetProperty, null, obj, [| v |]) |> ignore Now, you can use the operators to dynamically query data as follows: type Record1 = { Length : int; mutable Values : int list } let obj1 = box [1;2;3] let obj2 = box { Length = 4; Values = [3;4;5;7] } let n1 : int = obj1 Length let n2 : int = obj2 Length let valuesOld : int list = obj2 Values Here, both obj1 and obj2 have type obj, but you can do dynamic lookups of the properties Length and Values using the operator. Of course, these uses aren t strongly statically typed this is why you need the type annotations : int and : int list to indicate the return type of the operation. Given the earlier definition of the ( <-) operator, you can also set a property dynamically as follows:
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