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Using Active patterns

Shows how to use an active pattern that will try to determine the format of an input value.

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open System

// definition of the active pattern
let (|Bool|Int|Float|String|) input =
    // attempt to parse a bool
    let sucess, res = Boolean.TryParse input
    if sucess then Bool(res)
    else 
        // attempt to parse an int
        let sucess, res = Int32.TryParse input
        if sucess then Int(res)
        else
            // attempt to parse a float (Double)
            let sucess, res = Double.TryParse input
            if sucess then Float(res)
            else String(input)

// function to print the results by pattern
// matching over the active pattern
let printInputWithType input =
    match input with
    | Bool b -> printfn "Boolean: %b" b
    | Int i -> printfn "Integer: %i" i
    | Float f -> printfn "Floating point: %f" f
    | String s -> printfn "String: %s" s

// print the results    
printInputWithType "true"
printInputWithType "12"
printInputWithType "-12.1"
namespace System
Multiple items
type String =
  new : value:char -> string + 7 overloads
  member Chars : int -> char
  member Clone : unit -> obj
  member CompareTo : value:obj -> int + 1 overload
  member Contains : value:string -> bool
  member CopyTo : sourceIndex:int * destination:char[] * destinationIndex:int * count:int -> unit
  member EndsWith : value:string -> bool + 2 overloads
  member Equals : obj:obj -> bool + 2 overloads
  member GetEnumerator : unit -> CharEnumerator
  member GetHashCode : unit -> int
  ...

Full name: System.String

--------------------
String(value: nativeptr<char>) : unit
String(value: nativeptr<sbyte>) : unit
String(value: char []) : unit
String(c: char, count: int) : unit
String(value: nativeptr<char>, startIndex: int, length: int) : unit
String(value: nativeptr<sbyte>, startIndex: int, length: int) : unit
String(value: char [], startIndex: int, length: int) : unit
String(value: nativeptr<sbyte>, startIndex: int, length: int, enc: Text.Encoding) : unit
val input : string
val sucess : bool
val res : bool
type Boolean =
  struct
    member CompareTo : obj:obj -> int + 1 overload
    member Equals : obj:obj -> bool + 1 overload
    member GetHashCode : unit -> int
    member GetTypeCode : unit -> TypeCode
    member ToString : unit -> string + 1 overload
    static val TrueString : string
    static val FalseString : string
    static member Parse : value:string -> bool
    static member TryParse : value:string * result:bool -> bool
  end

Full name: System.Boolean
Boolean.TryParse(value: string, result: byref<bool>) : bool
val res : int
type Int32 =
  struct
    member CompareTo : value:obj -> int + 1 overload
    member Equals : obj:obj -> bool + 1 overload
    member GetHashCode : unit -> int
    member GetTypeCode : unit -> TypeCode
    member ToString : unit -> string + 3 overloads
    static val MaxValue : int
    static val MinValue : int
    static member Parse : s:string -> int + 3 overloads
    static member TryParse : s:string * result:int -> bool + 1 overload
  end

Full name: System.Int32
Int32.TryParse(s: string, result: byref<int>) : bool
Int32.TryParse(s: string, style: Globalization.NumberStyles, provider: IFormatProvider, result: byref<int>) : bool
val res : float
type Double =
  struct
    member CompareTo : value:obj -> int + 1 overload
    member Equals : obj:obj -> bool + 1 overload
    member GetHashCode : unit -> int
    member GetTypeCode : unit -> TypeCode
    member ToString : unit -> string + 3 overloads
    static val MinValue : float
    static val MaxValue : float
    static val Epsilon : float
    static val NegativeInfinity : float
    static val PositiveInfinity : float
    ...
  end

Full name: System.Double
Double.TryParse(s: string, result: byref<float>) : bool
Double.TryParse(s: string, style: Globalization.NumberStyles, provider: IFormatProvider, result: byref<float>) : bool
val printInputWithType : input:string -> unit

Full name: Script.printInputWithType
active recognizer Bool: string -> Choice<bool,int,float,string>

Full name: Script.( |Bool|Int|Float|String| )
val b : bool
val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn
active recognizer Int: string -> Choice<bool,int,float,string>

Full name: Script.( |Bool|Int|Float|String| )
val i : int
active recognizer Float: string -> Choice<bool,int,float,string>

Full name: Script.( |Bool|Int|Float|String| )
val f : float
Multiple items
active recognizer String: string -> Choice<bool,int,float,string>

Full name: Script.( |Bool|Int|Float|String| )

--------------------
type String =
  new : value:char -> string + 7 overloads
  member Chars : int -> char
  member Clone : unit -> obj
  member CompareTo : value:obj -> int + 1 overload
  member Contains : value:string -> bool
  member CopyTo : sourceIndex:int * destination:char[] * destinationIndex:int * count:int -> unit
  member EndsWith : value:string -> bool + 2 overloads
  member Equals : obj:obj -> bool + 2 overloads
  member GetEnumerator : unit -> CharEnumerator
  member GetHashCode : unit -> int
  ...

Full name: System.String

--------------------
String(value: nativeptr<char>) : unit
String(value: nativeptr<sbyte>) : unit
String(value: char []) : unit
String(c: char, count: int) : unit
String(value: nativeptr<char>, startIndex: int, length: int) : unit
String(value: nativeptr<sbyte>, startIndex: int, length: int) : unit
String(value: char [], startIndex: int, length: int) : unit
String(value: nativeptr<sbyte>, startIndex: int, length: int, enc: Text.Encoding) : unit
val s : string

More information

Link:http://fssnip.net/B
Posted:6 years ago
Author:Robert Pickering
Tags: active pattern