Why does the Haskell PVP describe new functions as non-breaking?

Question

Let's say I have the following code

module A where

  x :: Int 
  x = 5

module Main where

  import A
  import Lib
  
  main :: IO ()
  main = print (x + y)

and in an external library

module Lib where

  y :: Int
  y = 10

All well and good, the library is at v0.1.0.0 and I include it with bounds mylib == 0.1.*

Now, the name x :: Int is added to the export list of MyLib and a new release is out. It's version is v0.1.1.0, as suggested in the PVP flowchart:

In this case, only [...] functions [...] were added to the library's exported interface. No breakage [...] could result

How is this assertion correct? Surely my code no longer compiles. Since it doesn't know where to pull x from.

It's not mylib's interface alone that causes the breakage, which is what a major-number bump would indicate. People not using A won't see a problem with the introduction of x. (I.e, it's not mylib's fault you aren't using import qualified A :) ) — chepner
– chepner, Commented Feb 23, 2024 at 21:51

Noughtmare · Accepted Answer · 2024-02-23 21:58:33Z

6

You can prevent breakage with a defensive importing style. The Haskell wiki has an article about it:

We recommend to focus on the following two forms of import:

import qualified Very.Special.Module as VSM
import Another.Important.Module (printf, (<|>), )

instead of

import Very.Special.Module
import Another.Important.Module hiding (open, close, )

And the PVP rules are one of the reasons they advocate for the explicit style.

answered Feb 23, 2024 at 21:33

Noughtmare

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