Build data constructor using generics in Haskell

Question

How to construct a value by given constructor name and constructor arguments. For example, having the following data type

{-# LANGUAGE DeriveGeneric #-}

data Test = Foo | Bar Int | Baz String Int deriving (Generic, Show)

I would have such function

make "Foo" -- > Foo
make "Bar" 1 -- > Bar 1
make "Baz" "hi" 777 -- > Baz "hi" 777

The solution in this article is pretty close, but it works for data types only with single data constructor.

EDIT

So why do I want to do this? I have a BERT-encoded data that comes from the client. I can decode it to the Terms from the bert package. I want to have something like this

case x of
  Foo -> do something
  Bar x -> do something with x

and so on instead of

case x of
  TupleTerm y ->
    case y of
      [AtomTerm "foo"] -> do something
      [AtomTerm "bar", IntTerm x] -> do something with x

EDIT2

I concluded, that I was going in the wrong way.

Well the problem with this is: what would the signature of the make function be? String -> Test, String -> Int -> Test? String -> String -> Int -> Test? — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Aug 16, 2018 at 17:54
well even if that was possible, then how would you process this? What would the type of make some_string be? Since it depends on the value of some_string, it has no specific value, hence type contracts could no longer get satisfied. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Aug 16, 2018 at 18:10
I have a BERT encoded data, that comes from the web browser, and I'm trying to turn it into the plain Haskell values. It is annoying to match BERT Terms every time. — xafizoff
– xafizoff, Commented Aug 16, 2018 at 18:30
Now I think there should be a better way for such transformations. I.e. transform :: (Generic t, FromBert (Rep t)) => Term -> Maybe t — xafizoff
– xafizoff, Commented Aug 16, 2018 at 18:44

John F. Miller · Accepted Answer · 2018-08-16 18:53:49Z

1

"I have a BERT encoded data, that comes from the web browser"

There is not a reasonable way to solve the problem as you have posed it. However, if you are parsing data and you want to turn it into a Haskell data structure, the place to select the constructor is within the parser. Something like:

make :: Parser Test
make = do
  tag <- parse string
  case tag of
    "Foo" -> Foo
    "Bar" -> Bar <$> int
    "Baz" -> Baz <$> string <*> int

sting :: Parser String
int   :: Parser Int

answered Aug 16, 2018 at 18:53

John F. Miller

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1 Comment

xafizoff Over a year ago

Yes, I agree. But I want to do this in a generic way.

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