Basic arg parser (#216)

WoofWare.Myriad.Plugins.Attributes/ArgParserAttributes.fs

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+namespace WoofWare.Myriad.Plugins
+
+open System
+
+/// Attribute indicating a record type to which the "build arg parser" Myriad
+/// generator should apply during build.
+///
+/// If you supply isExtensionMethod = true, you will get extension methods.
+/// These can only be consumed from F#, but the benefit is that they don't use up the module name
+/// (since by default we create a module called "{TypeName}").
+type ArgParserAttribute (isExtensionMethod : bool) =
+    inherit Attribute ()
+
+    /// The default value of `isExtensionMethod`, the optional argument to the ArgParserAttribute constructor.
+    static member DefaultIsExtensionMethod = false
+
+    /// Shorthand for the "isExtensionMethod = false" constructor; see documentation there for details.
+    new () = ArgParserAttribute ArgParserAttribute.DefaultIsExtensionMethod
+
+/// Attribute indicating that this field shall accumulate all unmatched args,
+/// as well as any that appear after a bare `--`.
+type PositionalArgsAttribute () =
+    inherit Attribute ()
+
+/// Attribute indicating that this field shall have a default value derived
+/// from calling an appropriately named static method on the type.
+///
+/// This attribute can only be placed on fields of type `Choice<_, _>` where both type parameters
+/// are the same.
+/// After a successful parse, the value is Choice1Of2 if the user supplied an input,
+/// or Choice2Of2 if the input was obtained by calling the default function.
+///
+/// The static method we call for field `FieldName : 'a` is `DefaultFieldName : unit -> 'a`.
+type ArgumentDefaultFunctionAttribute () =
+    inherit Attribute ()
+
+/// Attribute indicating that this field shall have a default value derived
+/// from an environment variable (whose name you give in the attribute constructor).
+///
+/// This attribute can only be placed on fields of type `Choice<_, _>` where both type parameters
+/// are the same.
+/// After a successful parse, the value is Choice1Of2 if the user supplied an input,
+/// or Choice2Of2 if the input was obtained by pulling a value from `Environment.GetEnvironmentVariable`.
+type ArgumentDefaultEnvironmentVariableAttribute (envVar : string) =
+    inherit Attribute ()
+
+/// Attribute indicating that this field shall have the given help text, when `--help` is invoked
+/// or when a parse error causes us to print help text.
+type ArgumentHelpTextAttribute (helpText : string) =
+    inherit Attribute ()
+
+/// Attribute indicating that this field should be parsed with a ParseExact method on its type.
+/// For example, on a TimeSpan field, with [<ArgumentParseExact @"hh\:mm\:ss">], we will call
+/// `TimeSpan.ParseExact (s, @"hh\:mm\:ss", CultureInfo.CurrentCulture).
+type ParseExactAttribute (format : string) =
+    inherit Attribute ()
+
+/// Attribute indicating that this field should be parsed in the invariant culture, rather than the
+/// default current culture.
+/// For example, on a TimeSpan field, with [<InvariantCulture>] and [<ArgumentParseExact @"hh\:mm\:ss">], we will call
+/// `TimeSpan.ParseExact (s, @"hh\:mm\:ss", CultureInfo.InvariantCulture).
+type InvariantCultureAttribute () =
+    inherit Attribute ()