Will GHC strip out unused imports?

Question

When I don’t use certain imports from the main function (which should be easily detectable), will GHC strip out the corresponding bindings?

Example:

import Text.Parsec (parse)
import My.Testframework (test)

main = parse …

tests = test …

Will My.Testframework be linked in the executable?

Answer 1

I did this simple test:

import Data.Map as M
import Data.Text as T

main = do
  let m = M.fromList [(1, 2), (3, 4)]
  putStrLn $ show m

and got:

$ ghc imports.hs
$ $ ls -l imports
-rwxr-xr-x+ 1 erik  staff  1583112 May 17 10:56 imports

---

I then commented out the unused Data.Text import:

import Data.Map as M
-- import Data.Text as T

main = do
  let m = M.fromList [(1, 2), (3, 4)]
  putStrLn $ show m

and got this:

$ ghc imports.hs 
$ ls -l imports
-rwxr-xr-x+ 1 erik  staff  1583112 May 17 10:56 imports

Nothing changed, so I must assume Data.Text was already ignored since it was unused.

---

I then tried 2 more variants.

Leaving in the Data.Map import and the fromList call, but not using the value in m:

import Data.Map as M
-- import Data.Text as T

main = do
  let m = M.fromList [(1, 2), (3, 4)]
  putStrLn "hello"

which got me:

$ ghc imports.hs 
$ ls -l imports
-rwxr-xr-x+ 1 erik  staff  1505292 May 17 10:56 imports

---

I then also removed the construction of m using fromList:

import Data.Map as M
-- import Data.Text as T

main = do
  putStrLn "hello"

which got me:

$ ghc imports.hs 
$ ls -l imports
-rwxr-xr-x+ 1 erik  staff  1505284 May 17 10:56 imports

— the size barely changed.

So my interpretation is that even if you're importing a library and using it superficially, but the use itself is unused, GHC will not link libraries whose imports are unused.