instructure/canvas-lms · error · Errors::InvalidTypeCast

type_cast Proc must take exactly one argument for setting `#

Error message

type_cast Proc must take exactly one argument for setting `#{setting_name}`

What it means

Guard in Canvas::Operations::BaseConcerns::Settings#validate_type_cast!: a type_cast declared as a Proc for a canvas operation setting does not take exactly one argument, so Errors::InvalidTypeCast is raised at definition time. It's a developer-facing config validation, not a runtime data error.

Solutions

Rewrite the Proc to take exactly one argument and bind other values via closure
Move extra conversion parameters into the closure body, not the signature
Use a simple Symbol cast if no custom logic is needed

Example fix

// before
setting(:port, default: 80, type_cast: ->(v, base) { v.to_i(base) })
// after
setting(:port, default: 80, type_cast: ->(v) { v.to_i })

Defensive patterns

Strategy: validation

Validate before calling

raise 'proc must take 1 arg' unless type_cast.is_a?(Proc) && type_cast.arity == 1

Try / catch

rescue CanvasOperations::Errors::InvalidTypeCast
  value = default
end

Prevention

Write casts as single-parameter lambdas
Bind extra values via closure, not extra parameters
Remember proc {} has loose arity; prefer ->(v) { }

When it happens

Trigger: setting(:x, default: 0, type_cast: ->(v, base=10) { v.to_i(base) }) or a zero-arg proc like type_cast: -> { nil }.

Common situations: Reusing multi-argument converters; lambda vs proc arity quirks when writing -> without parameters; wrapping methods that have optional extra params.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/93e2bfc02ef02f44. Report an issue: GitHub.

Appendix: source

Thrown at lib/canvas_operations/base_concerns/settings.rb:134

        #
        # Always returns the setting for the operation's current shard's cluster.
        define_method(setting_name) do
          self.class.send(setting_name, cluster:)
        end
        protected setting_name
      end

      def validate_type_cast!(type_cast, setting_name)
        case type_cast
        when Symbol
          # Setting values are always Strings, so ensure the type_cast method exists on String
          unless SETTINGS_TYPE_INSTANCE.respond_to?(type_cast)
            raise Errors::InvalidTypeCast,
                  "Unsupported type_cast `#{type_cast}` for setting `#{setting_name}`. #{SETTINGS_TYPE_INSTANCE.class} does not respond to `#{type_cast}`."
          end
        when Proc
          unless type_cast.arity == 1
            raise Errors::InvalidTypeCast, "type_cast Proc must take exactly one argument for setting `#{setting_name}`"
          end
        else
          raise Errors::InvalidTypeCast, "Unsupported type_cast `#{type_cast}` for setting `#{setting_name}`"
        end
      end

      def settings_key(setting_name, cluster:)
        "clusters/#{cluster}/#{operation_name}/#{setting_name}"
      end
    end
  end
end

View on GitHub (pinned to 1c9f0bb801)