将数据构造函数映射到类型

Question

I'm creating an application that allows an admin to build a form, which a user can fill out. Questions can be of different types. Each kind of question corresponds to a type of response data.

Is it possible to encode this at the type level? How would you organize this?

data QuestionType = EmailText | PlainText | Numeric

-- this doesn't have any response information, it's the metadata
-- about the question itself.
data Question = { id :: ID, label :: Text, questionType :: QuestionType }

data Answer = { questionID :: ID, response :: Response }

-- I would like to map those question types to different response types
data Response = ???
-- EmailText => Text
-- PlainText => Text
-- Numeric   => Int

I've thought about Type Families, which would work perfectly except that I want to map from different data constructors to different types, and type families would require separate types for each one.

This would be a good fit for a single ADT, with the response information included in each constructor, but I need to be able to deal with question types independently from responses.

How should I approach this?

Answer 1

I've not completely understood what you exactly want, but maybe this can be a starting point:

{-# LANGUAGE DataKinds, GADTs #-}

data Response (qt :: QuestionType) where
  RPlainText :: Text -> Response PlainText
  REmailText :: Text -> Response EmailText
  RNumeric :: Int -> Response Numeric

data Answer qt = Answer {questionID :: ID, response :: Response qt}

If you do not want the qt argument in Answer qt , you probably need existential types to hide that, but at that point you'll probably want to link it with the questions somehow.

Answer 2

Having types depend on values is exactly what dependent types are for. Unfortunately, Haskell doesn't have dependent types. However, you can use a sum type to capture the possible response types:

data Response = ResponseText Text | ResponseInt Int