Static Smart Constructors with Double Splices

I plan on using this technique in at least one library, and so thought it would be worth giving a name and a page dedicated to it, so it can be linked to, instead of explained all over again.

Sometimes you’re making a DSL and you need a way to make a smart constructor that is static. You want to validate a value at compile-time (possibly because the validation logic is harder to do in the type-system or not ergonomic enough), and also return the run-time value, without having to lift the whole structure from TH-land to real-land. Perhaps your data type is a funky GADT with functions in it, and therefore has no Lift instance anyway!

One pattern to achieve the desired result is using a double splice.

Here’s a really trivial example with strings:

> :t $$($$(notNull [|| thename ||]))
$$($$(notNull [|| thename ||])) :: NonEmptyString
> $$($$(notNull [|| thename ||]))
"Hello!"

> :t $$($$(notNull [|| thename_empty ||]))

<interactive>:1:4: error:
    • Exception when trying to run compile-time code:
        String is not empty.

Note that the notNull validator is well-typed (which is commonly lacking in Template-Haskell, but here works well):

> :t notNull
notNull :: Q (TExp String) -> Q (TExp (Q (TExp NonEmptyString)))

The above uses typed splices $$( ... ) and typed quotes [|| ... ||]. However, you can implement the same technique with regular $(..) and [| .. |] if typed splices are too long for you.¹

Here is the implementation:

{-# LANGUAGE TemplateHaskell #-}
import Language.Haskell.TH
import Language.Haskell.TH.Instances ()
import Language.Haskell.TH.Syntax
import Language.Haskell.TH.Lift ()

-- Here is a simple validator.

valueFine :: String -> Bool
valueFine = not . null

-- The static smart constructor with the nested expressions returned.

notNull :: Q (TExp String) -> Q (TExp (Q (TExp NonEmptyString)))
notNull q = do
  TExp expr <- q
  let name = 'NonEmptyString
  [|| if valueFine $$(q)
      then TExp <$> (appE (conE name) (pure expr))
      else error "String is not empty." ||]

-- A smart constructor.

data NonEmptyString = NonEmptyString String
 deriving (Show)

-- Some example values.

thename :: [Char]
thename = "Hello!"

thename_empty :: [Char]
thename_empty = ""

Thanks to Michael Sloan for the idea to have an expression return another expression.