Introduce AST error compiler

It makes message compiler a specific case of a generic error compiler,
just like new ast error compiler.

References dry-rb/dry-validation#604

lib/dry/schema/ast_error_compiler.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require 'dry/schema/error_compiler'
+require 'dry/schema/ast_error_set'
+
+module Dry
+  module Schema
+    # Compiles rule results AST into machine-readable format
+    #
+    # @api private
+    class AstErrorCompiler < ErrorCompiler
+      # @api private
+      def call(ast)
+        AstErrorSet[ast]
+      end
+
+      def with(*args)
+        self
+      end
+    end
+  end
+end

lib/dry/schema/ast_error_set.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require 'dry/schema/error_set'
+
+module Dry
+  module Schema
+    # A set of AST errors used to generate machine-readable errors
+    #
+    # @see Result#message_set
+    #
+    # @api public
+    class AstErrorSet < ErrorSet
+      include Enumerable
+
+      private
+
+      # @api private
+      def errors_map(errors = self.errors)
+        errors
+      end
+    end
+  end
+end

lib/dry/schema/config.rb

@@ -36,6 +36,16 @@ class Config
       # @api public
       setting(:types, Dry::Types)
 
+      # @!method error_compiler
+      #
+      # Return configured error_compiler
+      #
+      # @return [Dry::Schema::ErrorCompiler]
+      #
+      # @api public
+      setting(:error_compiler, :message)
+
+
       # @!method messages
       #
       # Return configuration for message backend

lib/dry/schema/error_compiler.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module Dry
+  module Schema
+    # Compiles rule results AST into some type of errors
+    #
+    # @api private
+    class ErrorCompiler
+      # @api private
+      def call(_ast)
+        raise NotImplementedError
+      end
+
+      def with(_args)
+        raise NotImplementedError
+      end
+    end
+  end
+end

lib/dry/schema/error_set.rb

@@ -0,0 +1,110 @@
+# frozen_string_literal: true
+
+require 'dry/equalizer'
+
+module Dry
+  module Schema
+    # A set of generic errors
+    #
+    # @see Result#message_set
+    #
+    # @api public
+    class ErrorSet
+      include Enumerable
+      include Dry::Equalizer(:errors, :options)
+
+      # A list of compiled errors
+      #
+      # @return [Array<Any>]
+      attr_reader :errors
+
+      # Options hash
+      #
+      # @return [Hash]
+      attr_reader :options
+
+      # @api private
+      def self.[](errors, options = EMPTY_HASH)
+        new(errors, options)
+      end
+
+      # @api private
+      def initialize(errors, options = EMPTY_HASH)
+        @errors = errors
+        @options = options
+      end
+
+      # Iterate over errors
+      #
+      # @example
+      #   result.errors.each do |message|
+      #     puts message.text
+      #   end
+      #
+      # @return [Array]
+      #
+      # @api public
+      def each(&block)
+        return self if empty?
+        return to_enum unless block
+
+        errors.each(&block)
+      end
+
+      # Dump message set to a hash
+      #
+      # @return [Hash<Symbol=>Array<String>>]
+      #
+      # @api public
+      def to_h
+        @to_h ||= errors_map
+      end
+      alias_method :to_hash, :to_h
+
+      # Get a list of errors for the given key
+      #
+      # @param [Symbol] key
+      #
+      # @return [Array<String>]
+      #
+      # @api public
+      def [](key)
+        to_h[key]
+      end
+
+      # Get a list of errors for the given key
+      #
+      # @param [Symbol] key
+      #
+      # @return [Array<String>]
+      #
+      # @raise KeyError
+      #
+      # @api public
+      def fetch(key)
+        self[key] || raise(KeyError, "+#{key}+ error was not found")
+      end
+
+      # Check if an error set is empty
+      #
+      # @return [Boolean]
+      #
+      # @api public
+      def empty?
+        @empty ||= errors.empty?
+      end
+
+      # @api private
+      def freeze
+        to_h
+        empty?
+        super
+      end
+
+      # @api private
+      def errors_map(_errors)
+        raise NotImplementedError
+      end
+    end
+  end
+end

lib/dry/schema/extensions/hints/message_set_methods.rb

@@ -19,9 +19,9 @@ module MessageSetMethods
           attr_reader :failures
 
           # @api private
-          def initialize(messages, options = EMPTY_HASH)
+          def initialize(errors, options = EMPTY_HASH)
             super
-            @hints = messages.select(&:hint?)
+            @hints = errors.select(&:hint?)
             @failures = options.fetch(:failures, true)
           end
 
@@ -34,22 +34,23 @@ def initialize(messages, options = EMPTY_HASH)
           #
           # @api public
           def to_h
-            @to_h ||= failures ? messages_map : messages_map(hints)
+            @to_h ||= failures ? errors_map : errors_map(hints)
           end
           alias_method :to_hash, :to_h
 
           private
 
           # @api private
           def unique_paths
-            messages.uniq(&:path).map(&:path)
+            errors.uniq(&:path).map(&:path)
           end
 
           # @api private
-          def messages_map(messages = self.messages)
+          def errors_map(errors = self.errors)
             return EMPTY_HASH if empty?
 
-            messages.reduce(placeholders) { |hash, msg|
+            initialize_placeholders!
+            errors.reduce(placeholders) { |hash, msg|
               node = msg.path.reduce(hash) { |a, e| a.is_a?(Hash) ? a[e] : a.last[e] }
               (node[0].is_a?(::Array) ? node[0] : node) << msg.dump
               hash
@@ -61,7 +62,7 @@ def messages_map(messages = self.messages)
           # rubocop:disable Metrics/AbcSize
           # rubocop:disable Metrics/PerceivedComplexity
           def initialize_placeholders!
-            @placeholders = unique_paths.each_with_object(EMPTY_HASH.dup) { |path, hash|
+            @placeholders ||= unique_paths.each_with_object(EMPTY_HASH.dup) { |path, hash|
               curr_idx = 0
               last_idx = path.size - 1
               node = hash

lib/dry/schema/message_compiler.rb

@@ -2,6 +2,7 @@
 
 require 'dry/initializer'
 
+require 'dry/schema/error_compiler'
 require 'dry/schema/constants'
 require 'dry/schema/message'
 require 'dry/schema/message_set'
@@ -12,7 +13,7 @@ module Schema
     # Compiles rule results AST into human-readable format
     #
     # @api private
-    class MessageCompiler
+    class MessageCompiler < ErrorCompiler
       extend Dry::Initializer
 
       resolve_key_predicate = proc { |node, opts|
@@ -65,7 +66,7 @@ def call(ast)
         current_messages = EMPTY_ARRAY.dup
         compiled_messages = ast.map { |node| visit(node, EMPTY_OPTS.dup(current_messages)) }
 
-        MessageSet[compiled_messages, failures: options.fetch(:failures, true)]
+        MessageSet[compiled_messages.flatten, failures: options.fetch(:failures, true)]
       end
 
       # @api private

lib/dry/schema/message_compiler/visitor_opts.rb

@@ -1,12 +1,13 @@
 # frozen_string_literal: true
 
+require 'dry/schema/error_compiler'
 require 'dry/schema/constants'
 require 'dry/schema/message'
 
 module Dry
   module Schema
     # @api private
-    class MessageCompiler
+    class MessageCompiler < ErrorCompiler
       # Optimized option hash used by visitor methods in message compiler
       #
       # @api private