Significant GHCi speed regression with :module and `let` in GHC 8.2.1
I recently noticed that the performance of `doctest` in GHC 8.2.1 (see the corresponding [doctest issue](https://github.com/sol/doctest/issues/170)) can be much, much worse than in previous versions of 8.0.2. So bad, in fact, that a project with 865 `doctest` examples takes about three hours to complete in 8.2.1, whereas it would only take 8 seconds in 8.0.2.
To reproduce this issue in a fairly minimal way, you can use the following script to generate a file which simulates what `doctest` is doing:
```hs
-- GenExample.hs
module Main where
import Control.Monad
import System.Environment
import System.Exit
import System.IO
main :: IO ()
main = do
args <- getArgs
case args of
n:_ -> genExamples (read n)
_ -> do hPutStrLn stderr "usage: runghc GenExamples.hs <num-examples>"
exitWith $ ExitFailure 1
genExamples :: Int -> IO ()
genExamples nExamples = do
putStrLn ":l Foo"
ireplicateA_ nExamples genExample
genExample :: Int -> IO ()
genExample i = putStr $ unlines
[ ":m *Foo"
, "example : \"expr" ++ show i ++ "\""
, "let foo = it"
, "\"marker\""
, "let it = foo"
]
ireplicateA_ :: Applicative m => Int -> (Int -> m a) -> m ()
ireplicateA_ cnt0 f =
loop cnt0 0
where
loop cnt n
| cnt <= 0 = pure ()
| otherwise = f n *> (loop (cnt - 1) $! (n + 1))
```
You'll also need this file:
```hs
-- Foo.hs
module Foo where
example :: Char
example = 'a'
```
First, use `GenExample` to generate a GHCi script:
```
$ runghc GenExample.hs 500 > Example.script
```
Now you can run the script like so (using GHCi 8.0.2 as an example):
```
$ /opt/ghc/8.0.2/bin/ghci -ghci-script Example.script Foo.hs
```
With GHCi 8.0.2, this takes about three seconds. But with GHCi 8.2.1, this takes about 35 seconds!
issue