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"Guard" helper function for railway error handling

Defines a "guard" function for railway-style error handing which allows you to concisely verify a condition when handling errors using Choice<'T, 'Error>. It checks the condition and if it is false, returns Choice2Of2 with the specified error value. If the condition is true then it returns Choice1Of2 (). Loosely inspired by Swift 2.0's guard keyword.

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open ExtCore.Control

let succeed = Choice1Of2
let fail = Choice2Of2
let guard cond err = if cond then succeed () else fail err

[<Measure>] type GBP 

type VendingMachineState =
    { Items : Map<string, int * float<GBP>>
      Funds : float<GBP> }

type VendingMachineError =
    | InvalidCashAmount of float<GBP>
    | UnknownItem of string
    | OutOfStock of string
    | InsufficientFunds of float<GBP>

let findItem item vendingMachine =
    match vendingMachine.Items |> Map.tryFind item with
    | Some result -> succeed result
    | None        -> fail (UnknownItem item) 

let payIn cash vendingMachine = choice {
    do! guard (cash > 0.0<GBP>) (InvalidCashAmount cash)
    return { vendingMachine with Funds = vendingMachine.Funds + cash } }

let vend item vendingMachine = choice {
    let! (quantity, price) = findItem item vendingMachine
    do! guard (quantity > 0) (OutOfStock item)
    do! guard (vendingMachine.Funds >= price) (InsufficientFunds <| price - vendingMachine.Funds)
    return (item,
        { vendingMachine with
            Funds = vendingMachine.Funds - price 
            Items = vendingMachine.Items |> Map.add item (quantity - 1, price) }) }

let vendingMachine = 
    { Items =
        [("Candy bar", (4, 1.20<GBP>))
         ("Coke"     , (0, 0.80<GBP>))
         ("Crisps"   , (2, 0.55<GBP>))]
        |> Map.ofList
      Funds = 0.60<GBP> }

// note : vending machine is immutable: each of the transactions starts with the same initial state
// defined above and returns a new vending machine state (or error)
vendingMachine |> payIn 2.0<GBP> |> printfn "%A"
vendingMachine |> vend "Crisps" |> printfn "%A"
vendingMachine |> vend "Coke" |> printfn "%A"
vendingMachine |> vend "Diet coke" |> printfn "%A"
vendingMachine |> vend "Candy bar" |> printfn "%A"

namespace ExtCore

namespace ExtCore.Control

val succeed : arg0:'a -> Choice<'a,'b>

Full name: Script.succeed

union case Choice.Choice1Of2: 'T1 -> Choice<'T1,'T2>

val fail : arg0:'a -> Choice<'b,'a>

Full name: Script.fail

union case Choice.Choice2Of2: 'T2 -> Choice<'T1,'T2>

val guard : cond:bool -> err:'a -> Choice<unit,'a>

Full name: Script.guard

val cond : bool

val err : 'a

Multiple items
type MeasureAttribute =
inherit Attribute
new : unit -> MeasureAttribute

Full name: Microsoft.FSharp.Core.MeasureAttribute

--------------------
new : unit -> MeasureAttribute

[<Measure>]
type GBP

Full name: Script.GBP

type VendingMachineState =
{Items: Map<string,(int * float<GBP>)>;
Funds: float<GBP>;}

Full name: Script.VendingMachineState

VendingMachineState.Items: Map<string,(int * float<GBP>)>

Multiple items
module Map

from ExtCore.Collections

--------------------
module Map

from Microsoft.FSharp.Collections

--------------------
type Map<'Key,'Value (requires comparison)> =
  interface IEnumerable
  interface IComparable
  interface IEnumerable<KeyValuePair<'Key,'Value>>
  interface ICollection<KeyValuePair<'Key,'Value>>
  interface IDictionary<'Key,'Value>
  new : elements:seq<'Key * 'Value> -> Map<'Key,'Value>
  member Add : key:'Key * value:'Value -> Map<'Key,'Value>
  member ContainsKey : key:'Key -> bool
  override Equals : obj -> bool
  member Remove : key:'Key -> Map<'Key,'Value>
  ...

Full name: Microsoft.FSharp.Collections.Map<_,_>

--------------------
new : elements:seq<'Key * 'Value> -> Map<'Key,'Value>

Multiple items
val string : value:'T -> string

Full name: Microsoft.FSharp.Core.Operators.string

--------------------
type string = System.String

Full name: Microsoft.FSharp.Core.string

--------------------
type string<'Measure> = string

Full name: ExtCore.string<_>

Multiple items
val int : value:'T -> int (requires member op_Explicit)

Full name: Microsoft.FSharp.Core.Operators.int

--------------------
type int = int32

Full name: Microsoft.FSharp.Core.int

--------------------
type int<'Measure> = int

Full name: Microsoft.FSharp.Core.int<_>

Multiple items
val float : value:'T -> float (requires member op_Explicit)

Full name: Microsoft.FSharp.Core.Operators.float

--------------------
type float = System.Double

Full name: Microsoft.FSharp.Core.float

--------------------
type float<'Measure> = float

Full name: Microsoft.FSharp.Core.float<_>

VendingMachineState.Funds: float<GBP>

type VendingMachineError =
  | InvalidCashAmount of float<GBP>
  | UnknownItem of string
  | OutOfStock of string
  | InsufficientFunds of float<GBP>

Full name: Script.VendingMachineError

union case VendingMachineError.InvalidCashAmount: float<GBP> -> VendingMachineError

union case VendingMachineError.UnknownItem: string -> VendingMachineError

union case VendingMachineError.OutOfStock: string -> VendingMachineError

union case VendingMachineError.InsufficientFunds: float<GBP> -> VendingMachineError

val findItem : item:string -> vendingMachine:VendingMachineState -> Choice<(int * float<GBP>),VendingMachineError>

Full name: Script.findItem

val item : string

val vendingMachine : VendingMachineState

val tryFind : key:'Key -> table:Map<'Key,'T> -> 'T option (requires comparison)

Full name: Microsoft.FSharp.Collections.Map.tryFind

union case Option.Some: Value: 'T -> Option<'T>

val result : int * float<GBP>

union case Option.None: Option<'T>

val payIn : cash:float<GBP> -> vendingMachine:VendingMachineState -> Choice<VendingMachineState,VendingMachineError>

Full name: Script.payIn

val cash : float<GBP>

val choice : ChoiceBuilder

Full name: ExtCore.Control.WorkflowBuilders.choice

val vend : item:string -> vendingMachine:VendingMachineState -> Choice<(string * VendingMachineState),VendingMachineError>

Full name: Script.vend

val quantity : int

val price : float<GBP>

val add : key:'Key -> value:'T -> table:Map<'Key,'T> -> Map<'Key,'T> (requires comparison)

Full name: Microsoft.FSharp.Collections.Map.add

val vendingMachine : VendingMachineState

Full name: Script.vendingMachine

val ofList : elements:('Key * 'T) list -> Map<'Key,'T> (requires comparison)

Full name: Microsoft.FSharp.Collections.Map.ofList

val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn

Raw view Test code New version

More information

Link:	http://fssnip.net/rn
Posted:	9 years ago
Author:	Anton Tcholakov
Tags:	error handling , choice , extcore