Why can ghci see non-exported types and constructors? How can I fix it?

Question

I am a novice in Haskell. Here's some simple code:

module Src
(   -- The 'Answer' type isn't exported
    Shape(Circle), -- i.e 'Rectangle' data constructor isn't exported
    Point(..),
    area,
    nudge
) where

data Answer = Yes | No deriving (Show)

data Point = Point Float Float deriving (Show)

data Shape = Circle Point Float | Rectangle Point Point
    deriving (Show)

area :: Shape -> Float
area (Circle _ r) = pi * r ^ 2
area (Rectangle (Point x1 y1) (Point x2 y2)) = (abs $ x2 - x1) * (abs $ y2 - y1)

nudge::Shape->Float->Float->Shape
nudge (Rectangle(Point x1 y1)(Point x2 y2)) dx dy = Rectangle 
    (Point (x1 + dx) (y1 + dy)) (Point (x2 + dx) (y2 + dy))
nudge (Circle (Point x y) r) dx dy  = Circle (Point(x + dx) (y + dy)) r

I've hidden the Answer type and the Rectangle constructor. But when I load the Src.hs file, ghci still sees them:

ghci> :l src
[1 of 1] Compiling Src ( src.hs, interpreted )
Ok, modules loaded: Src.
ghci> let a = Yes
ghci> a
Yes
ghci> :t a
a :: Answer
ghci> let r = Rectangle (Point 0 0) (Point 100 100)
ghci> r
Rectangle (Point 0.0 0.0) (Point 100.0 100.0)
ghci> :t r
r :: Shape
ghci>

Why did this happen and how can I fix it?

You've hidden things from things that import the module, but you can't hide a file from itself! — Simon H

sepp2k sepp2k · Accepted Answer · 2014-12-18T14:05:12

Yes, when you load a file in GHCi like that, you can access anything defined in that file regardless of whether or not it is exported. This way expressions you write into GHCi behave exactly like they would inside the loaded file. So you can use GHCi to quickly test an expression that you mean to use inside the file or to quickly test a function defined in the file even if it is private.

If you want the code to behave as if imported from another file, you can use import instead of :l. Then it will only allow access to expported definitions.

Why can ghci see non-exported types and constructors? How can I fix it?

2 Answers