Pretty-print contracts in error messages.

Author: frewsxcv

lib/contracts.rb

@@ -116,22 +116,57 @@ def to_s
   # This function is used by the default #failure_callback method
   # and uses the hash passed into the failure_callback method.
   def self.failure_msg(data)
-    expected = Contracts::Formatters::Expected.new(data[:contract]).contract
-    position = Contracts::Support.method_position(data[:method])
+    indent_amount = 8
     method_name = Contracts::Support.method_name(data[:method])
 
+    # Header
     header = if data[:return_value]
                "Contract violation for return value:"
              else
                "Contract violation for argument #{data[:arg_pos]} of #{data[:total_args]}:"
              end
 
-    %{#{header}
-        Expected: #{expected},
-        Actual: #{data[:arg].inspect}
-        Value guarded in: #{data[:class]}::#{method_name}
-        With Contract: #{data[:contracts]}
-        At: #{position} }
+    # Expected
+    expected_prefix = "Expected: "
+    expected_value = Contracts::Support.indent_string(
+      Contracts::Formatters::Expected.new(data[:contract]).contract.pretty_inspect,
+      expected_prefix.length
+    ).strip
+    expected_line = expected_prefix + expected_value + ","
+
+    # Actual
+    actual_prefix = "Actual: "
+    actual_value = Contracts::Support.indent_string(
+      data[:arg].pretty_inspect,
+      actual_prefix.length
+    ).strip
+    actual_line = actual_prefix + actual_value
+
+    # Value guarded in
+    value_prefix = "Value guarded in: "
+    value_value = "#{data[:class]}::#{method_name}"
+    value_line = value_prefix + value_value
+
+    # Contract
+    contract_prefix = "With Contract: "
+    contract_value = data[:contracts].to_s
+    contract_line = contract_prefix + contract_value
+
+    # Position
+    position_prefix = "At: "
+    position_value = Contracts::Support.method_position(data[:method])
+    position_line = position_prefix + position_value
+
+    header +
+      "\n" +
+      Contracts::Support.indent_string(
+        [expected_line,
+         actual_line,
+         value_line,
+         contract_line,
+         position_line].join("\n"),
+        indent_amount
+      )
   end
 
   # Callback for when a contract fails. By default it raises

lib/contracts/formatters.rb

@@ -1,3 +1,5 @@
+require "pp"
+
 module Contracts
   # A namespace for classes related to formatting.
   module Formatters
@@ -31,7 +33,7 @@ def hash_contract(hash)
       # Formats Array contracts.
       def array_contract(array)
         @full = true
-        array.map { |v| InspectWrapper.create(contract(v), @full) }.inspect
+        array.map { |v| InspectWrapper.create(contract(v), @full) }
       end
     end
 

lib/contracts/support.rb

@@ -42,6 +42,13 @@ def eigenclass?(target)
           target <= eigenclass_of(Object)
       end
 
+      def indent_string(string, amount)
+        string.gsub(
+          /^(?!$)/,
+          (string[/^[ \t]/] || " ") * amount
+        )
+      end
+
       private
 
       # Module eigenclass can be detected by its ancestor chain

spec/contracts_spec.rb

@@ -637,6 +637,28 @@ def delim(match)
       end.to raise_error(ContractError, not_s(delim "String or Symbol"))
     end
 
+    it "should wrap and pretty print for long param contracts" do
+      expect do
+        @o.long_array_param_contracts(true)
+      end.to(
+        raise_error(
+          ParamContractError,
+          /\[\(String or Symbol\),\n                   \(String or Symbol\),/
+        )
+      )
+    end
+
+    it "should wrap and pretty print for long return contracts" do
+      expect do
+        @o.long_array_return_contracts
+      end.to(
+        raise_error(
+          ReturnContractError,
+          /\[\(String or Symbol\),\n                   \(String or Symbol\),/
+        )
+      )
+    end
+
     it "should not contain Contracts:: module prefix" do
       expect do
         @o.double("bad")

spec/fixtures/fixtures.rb

@@ -152,6 +152,30 @@ def array_complex_contracts(data)
   def nested_array_complex_contracts(data)
   end
 
+  Contract [
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol]
+  ] => nil
+  def long_array_param_contracts(data)
+  end
+
+  Contract C::None => [
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol],
+    C::Or[String, Symbol]
+  ]
+  def long_array_return_contracts
+  end
+
   Contract Proc => C::Any
   def do_call(&block)
     block.call