GHCinception:
Running GHCi in GHCi

mgsloan

I'm happy to announce that you can now easily load GHC into GHCi! I've been using this for about a month now, and for me it makes GHC development far more pleasant than using make. This can often lead to iteration times of only 10-30 seconds to try out some modified behavior.

For me, GHCi usage is crucial to efficient Haskell development. One larger project I work on takes a whopping 15 minutes to build, whereas loading the same code in GHCi takes a little bit less than 1 minute.

GHCi is quite a marvelous thing. Despite being an interpreter, it can run complicated programs quite quickly and correctly – I typically barely notice a difference in performance. One of the main reasons for this is that it still uses the compiled code for your dependencies. It also lets you conveniently query lots of information and try things out in a REPL.

How to load GHC into GHCi

If you have a recent checkout of GHC, and have built the GHC stage 2 compiler located at /inplace/bin/ghc-stage2, then you can do the following to load GHC into GHCi:

$ ./utils/ghc-in-ghci/run.sh -j8

Tweak the N in -jN as is suitable for your system – it indicates the degree of module-level build parallelism. The number of logical cores you have is a reasonable guideline.

This should result in GHCi loading up the code for GHC:

mgsloan@treetop:~/oss/haskell/ghc$ ./utils/ghc-in-ghci/run.sh -fobject-code -j8
+ export _GHC_TOP_DIR=./inplace/lib
+ exec ./inplace/bin/ghc-stage2 --interactive -ghci-script ./utils/ghc-in-ghci/settings.ghci -ghci-script ./utils/ghc-in-ghci/load-main.ghci -odir ./ghci-tmp -hidir ./ghci-tmp +RTS -A128m -RTS -fobject-code
GHCi, version 8.7.20180718: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/mgsloan/.ghci
package flags have changed, resetting and loading new packages...
Loaded GHCi configuration from ./utils/ghc-in-ghci/settings.ghci
[  1 of 493] Compiling GhcPrelude       ( compiler/utils/GhcPrelude.hs, ghci-tmp/GhcPrelude.o )
[  2 of 493] Compiling FiniteMap        ( compiler/utils/FiniteMap.hs, ghci-tmp/FiniteMap.o )
[  3 of 493] Compiling FastMutInt       ( compiler/utils/FastMutInt.hs, ghci-tmp/FastMutInt.o )
[  4 of 493] Compiling FastFunctions    ( compiler/utils/FastFunctions.hs, ghci-tmp/FastFunctions.o )
[  5 of 493] Compiling Exception        ( compiler/utils/Exception.hs, ghci-tmp/Exception.o )
[  6 of 493] Compiling EnumSet          ( compiler/utils/EnumSet.hs, ghci-tmp/EnumSet.o )
[  7 of 493] Compiling Encoding         ( compiler/utils/Encoding.hs, ghci-tmp/Encoding.o )
...

This takes a while to load, since it's loading 493 modules. On my machine it clocks in at around 8 minutes. The main reason that this takes so long is that it needs to do object code generation. I bet it'd also load faster if I wasn't using an unoptimized devel2 flavour build of ghc-stage2. This flavour includes potentially expensive assertions.

Once it has finished loading, you can use GHCi to ask for information as usual! For example, let's take a look at the datatype for a core expression:

Ok, 493 modules loaded.
Loaded GHCi configuration from ./utils/ghc-in-ghci/load-main.ghci
λ :m + CoreSyn
λ :info Expr
data Expr b
  = Var Var.Id
  | Lit Literal.Literal
  | App (Expr b) (Arg b)
  | Lam b (Expr b)
  | Let (Bind b) (Expr b)
  | Case (Expr b) b TyCoRep.Type [Alt b]
  | Cast (Expr b) TyCoRep.Coercion
  | Tick (Tickish Var.Id) (Expr b)
  | Type TyCoRep.Type
  | Coercion TyCoRep.Coercion

Beautiful! While -fobject-code does take quite a while to do an initial load, subsequent loads can be very fast. For me, with no code changes, it only takes about 7 seconds.

How to run GHCi within GHCi

The ghci script sets up some default arguments:

:set args --interactive -ghci-script utils/ghc-in-ghci/inner.ghci

Due to these defaults, just running main brings up a nested GHCi:

λ main
GHCi, version 8.7.20180718: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/mgsloan/.ghci
Loaded GHCi configuration from utils/ghc-in-ghci/inner.ghci
Prelude [inner]> :{
Prelude| putStrLn $ unwords $
Prelude| "Wow, we're evaluating code" : replicate 2 "*inside* GHCi"
Prelude| :}
Wow, we're evaluating code *inside* GHCi *inside* GHCi
Prelude [inner]>

There's a different GHCi prompt there because the inner.ghci contains

:set prompt "%s [inner]> "

When using this for development, I found that it was helpful to distinguish the inner ghci prompt from the outer.

Modifying, reloading, and trying some new behavior

Lets say I want to change some type error messages. First, I make a change to the code:

diff --git a/compiler/typecheck/TcErrors.hs b/compiler/typecheck/TcErrors.hs
index ecb404289a..32d7cffaab 100644
--- a/compiler/typecheck/TcErrors.hs
+++ b/compiler/typecheck/TcErrors.hs
@@ -1894,11 +1894,11 @@ misMatchMsg ct oriented ty1 ty2
   where
     herald1 = conc [ "Couldn't match"
                    , if is_repr     then "representation of" else ""
-                   , if is_oriented then "expected"          else ""
+                   , if is_oriented then "wanted"            else ""
                    , what ]
     herald2 = conc [ "with"
                    , if is_repr     then "that of"           else ""
-                   , if is_oriented then ("actual " ++ what) else "" ]
+                   , if is_oriented then ("found " ++ what)  else "" ]
     padding = length herald1 - length herald2

     is_repr = case ctEqRel ct of { ReprEq -> True; NomEq -> False }

Then I do a reload, run the inner GHCi, and try it out:

λ :r
[ 22 of 493] Compiling Hooks[boot]      ( compiler/main/Hooks.hs-boot, ghci-tmp/Hooks.o-boot )
... Omitted about 40 boot modules that load really quickly ...
[377 of 493] Compiling TcErrors         ( compiler/typecheck/TcErrors.hs, ghci-tmp/TcErrors.o )
... A few more boot modules ...
Ok, 493 modules loaded.
λ

This only took about 8 seconds!!! Invoking the inner GHCi takes less than a second:

λ main
GHCi, version 8.7.20180718: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/mgsloan/.ghci
Loaded GHCi configuration from utils/ghc-in-ghci/inner.ghci
Prelude [inner]> "testing" ++ Nothing

<interactive>:1:14: error:
    • Couldn't match wanted type ‘[Char]’ with found type ‘Maybe a0’
    • In the second argument of ‘(++)’, namely ‘Nothing’
      In the expression: "testing" ++ Nothing
      In an equation for ‘it’: it = "testing" ++ Nothing
Prelude [inner]>

For me, this is a game changer for GHC development. In particular, this means that you can make a change and very rapidly try out the change in behavior. On my machine, it often takes less than 10 seconds to make a change and try it out. This is drastically faster than make – I'm not even going to try, but I'm pretty sure I'd be waiting around for a few minutes.

Changes made to GHC for this

I opened a couple patches to GHC that got merged yesterday:

Next steps

Can we go three layers deep? Does time slow down?

inception screenshot

A natural question is how deep can we go? Can GHCi run inside GHCi inside GHCi?! It turns out that it works directly, and the nesting works arbitrarily deep:

Prelude [inner]> :script utils/ghc-in-ghci/settings.ghci
package flags have changed, resetting and loading new packages...
Prelude [inner]> :load Main
Ok, 493 modules loaded.
Main [inner]> main
GHCi, version 8.7.20180718: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/mgsloan/.ghci
Loaded GHCi configuration from utils/ghc-in-ghci/inner.ghci
Prelude [inner]> :script utils/ghc-in-ghci/settings.ghci
package flags have changed, resetting and loading new packages...
Prelude [inner]> :load Main
Ok, 493 modules loaded.
Main [inner]> main
GHCi, version 8.7.20180718: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/mgsloan/.ghci
Loaded GHCi configuration from utils/ghc-in-ghci/inner.ghci
Prelude [inner]> 1 + 1
2
Prelude [inner]> :q
Leaving GHCi.
Main [inner]> :q
Leaving GHCi.
Main [inner]> :q
Leaving GHCi.
λ :q
Leaving GHCi.

4 layers deep of GHCi! This is kinda cheating, though, because it is reusing the existing object files, and so it isn't really running the compiler.

I've tried removing the object files and loading GHCi inside GHCi inside GHCi, and it took about 9 minutes to load about 170 modules. At that point, it exited with a panic:

<no location info>: error:
    ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20180718 for x86_64-unknown-linux):
        ASSERT failed!
  $c==_a81xD
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in ghc:Outputable
        pprPanic, called at compiler/utils/Outputable.hs:1223:5 in ghc:Outputable
        assertPprPanic, called at compiler/stgSyn/CoreToStg.hs:991:78 in ghc:CoreToStg

This assertion wouldn't have been caught by an optimized build. It would be interesting to dig into what's going on there.

On to the question about how much slower this interpreted GHC is. The initial layering of having an interpreted GHC compile GHC definitely produces a massive slowdown in compilation speed. However, I each subsequent nesting would not compound the slowdown, because it's running object code. Even if the byte-code interpreter was being used, it's not being interpreted (it's written in C). Since the actual interpreter implementation is not being interpreted, the slowdown would not compound.

So pretty much, GHCi nesting does not follow inception's rules of time slow down :-)

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