Haskell - Export data constructor

问题

I have this data on my Module Formula :

data Formula = Formula {
    typeFormula :: String, 
    nbClauses   :: Int,
    nbVars      :: Int,
    clauses     :: Clauses       
}

And I want to export it but I don't know the right syntax :

module Formula (
    Formula ( Formula ),
    solve
) where

Someone can tell me the right syntax please ?

回答1:

Some of your confusion is coming from having the same module name as the constructor you're trying to export.

module Formula (
    Formula ( Formula ),
    solve
) where

Should be

module Formula (
    Formula (..),
    solve
) where

Or

module Formula (
    module Formula ( Formula (..)),
    solve
) where

---

Your current export statement says, in the Module Forumla, export the Type Formula defined in the Module Formula and the function solve (that is in scope for the module, wherever it is defined))

The (..) syntax means, export all constructors for the preceding type. In your case, it is equivalent to the explicit

module Formula (
    Formula (typeFormula,nbClauses, nbVars,clauses),
    solve
) where

来源：https://stackoverflow.com/questions/47929556/haskell-export-data-constructor