factor out common error handling logic from the Block Directive parser

Author: tayloraswift

Package.resolved

@@ -0,0 +1,15 @@
+extension Markdown
+{
+    @frozen public
+    struct AnyOption:Markdown.BlockDirectiveOption
+    {
+        public
+        let rawValue:String
+
+        @inlinable public
+        init(rawValue:String)
+        {
+            self.rawValue = rawValue
+        }
+    }
+}

Sources/MarkdownAST/BlockElements/Markdown.AnyOption.swift

@@ -52,13 +52,13 @@ extension Markdown
 extension Markdown.BlockDirective:Markdown.BlockDirectiveType
 {
     @inlinable public
-    func configure(option:String, value:Markdown.SourceString) throws
+    func configure(option:Markdown.AnyOption, value:Markdown.SourceString)
     {
-        self.arguments.append((option, value.string))
+        self.arguments.append((option.rawValue, value.string))
     }
 
     @inlinable public
-    func append(_ element:Markdown.BlockElement) throws
+    func append(_ element:Markdown.BlockElement)
     {
         self.elements.append(element)
     }

Sources/MarkdownAST/BlockElements/Markdown.BlockDirective.swift

@@ -0,0 +1,9 @@
+import MarkdownABI
+
+extension Markdown
+{
+    public
+    protocol BlockDirectiveOption:RawRepresentable<String>, Sendable
+    {
+    }
+}

Sources/MarkdownAST/BlockElements/Markdown.BlockDirectiveOption.swift

@@ -4,9 +4,11 @@ import Sources
 extension Markdown
 {
     public
-    protocol BlockDirectiveType:BlockElement
+    protocol BlockDirectiveType<Option>:BlockElement
     {
-        func configure(option:String, value:SourceString) throws
+        associatedtype Option:BlockDirectiveOption
+
+        func configure(option:Option, value:SourceString) throws
         func append(_ element:BlockElement) throws
 
         /// Associates a source location with this block. For ideal formatting, this source range
@@ -17,7 +19,7 @@ extension Markdown
 extension Markdown.BlockDirectiveType where Self:Markdown.BlockContainer<Markdown.BlockElement>
 {
     public
-    func append(_ element:Markdown.BlockElement) throws
+    func append(_ element:Markdown.BlockElement)
     {
         self.elements.append(element)
     }

Sources/MarkdownAST/BlockElements/Markdown.BlockDirectiveType.swift

@@ -0,0 +1,12 @@
+extension Markdown
+{
+    @frozen public
+    enum NeverOption:BlockDirectiveOption
+    {
+        @inlinable public
+        init?(rawValue:String) { nil }
+
+        @inlinable public
+        var rawValue:String { fatalError() }
+    }
+}

Sources/MarkdownAST/BlockElements/Markdown.NeverOption.swift

@@ -0,0 +1,18 @@
+import MarkdownAST
+
+extension Markdown.BlockDirectiveType
+{
+    func configure(option:String, value:Markdown.SourceString, block:String) throws
+    {
+        guard
+        let option:Option = .init(rawValue: option)
+        else
+        {
+            throw Markdown.BlockDirectiveUnexpectedOptionError.init(
+                option: option,
+                block: block)
+        }
+
+        try self.configure(option: option, value: value)
+    }
+}

Sources/MarkdownParsing/Markdown.BlockDirectiveType (ext).swift

@@ -0,0 +1,17 @@
+import MarkdownAST
+
+extension Markdown
+{
+    struct BlockDirectiveUnexpectedOptionError:Error
+    {
+        let option:String
+        let block:String
+    }
+}
+extension Markdown.BlockDirectiveUnexpectedOptionError:CustomStringConvertible
+{
+    var description:String
+    {
+        "unexpected option '\(self.option)' for block directive '@\(self.block)'"
+    }
+}

Sources/MarkdownParsing/Markdown.BlockDirectiveUnexpectedOptionError.swift

@@ -107,7 +107,8 @@ extension Markdown.Parser
                         option: argument.name,
                         value: .init(
                             source: .init(from: argument.valueRange, in: source),
-                            string: argument.value))
+                            string: argument.value),
+                        block: block.name)
                 }
                 catch let error
                 {