#537 Recompilation check fails for TH

[Simon Peyton Jones]

The recompilation check only recompiles a module when
the *interface* of a module it imports changes. But
with Template Haskell, it may need to be recompiled
when the *implementation* changes.

Concrete example below. It's quite awkward to fix.

* Perhaps a module that contains any splices should be
recompiled always.
* Perhaps a module that exports any TH stuff (how
would we tell?) should be flagged as changed if
anything about it changes.

Simon

The following scenario reproduces this error
(thanks to Bulat Ziganshin bulatz@HotPOP.com):

1) create Main.hs containing code

module Main where
import Sub
main = print $x

and Sub.hs containing code

module Sub where
x = [| 1 |]

2) compile them with --make:

C:\!\Haskell\!>ghc --make -fth Main.hs
Chasing modules from: Main.hs
Compiling Sub ( ./Sub.hs, ./Sub.o )
Compiling Main ( Main.hs, Main.o )
Loading package base-1.0 ... linking ... done.
Loading package haskell98-1.0 ... linking ... done.
Loading package template-haskell-1.0 ... linking ...
done.
Linking ...

C:\!\Haskell\!>main.exe
1

3) now change Sub.hs to the following code:

module Sub where
x = [| 2 |]

4) and recompile program:

C:\!\Haskell\!>ghc --make -fth Main.hs
Chasing modules from: Main.hs
Compiling Sub ( ./Sub.hs, ./Sub.o )
Skipping Main ( Main.hs, Main.o )
Linking ...

C:\!\Haskell\!>main.exe
1

As you see, Main.hs is not recompiled despite the fact
that definition
of x is changed and now program must print "2"