GHC: ghc gives clang error, but ghc --make works - haskell

Question

Long story short: I have two .hs files, where Main.hs uses module Lib.hs
When I try to compile them as suggested by the book Real World Haskell, p 115, I got problems:
$ ghc -c Lib.hs
$ ghc -o main Main.hs Lib.o
[2 of 2] Compiling Main ( Main.hs, Main.o )
Linking main ...
duplicate symbol '_Lib_add1_info' in:
./Lib.o
Lib.o
duplicate symbol '_Lib_add1_closure' in:
./Lib.o
Lib.o
ld: 3 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
`clang' failed in phase `Linker'. (Exit code: 1)
But, when I simply use ghc --make, it works fine
$ ghc --make Main.hs
[1 of 2] Compiling Lib ( Lib.hs, Lib.o )
[2 of 2] Compiling Main ( Main.hs, Main.o )
Linking Main ...
$ ls -1
Lib.hi
Lib.hs
Lib.o
Main
Main.hi
Main.hs
Main.o
Can anyone explain why ghc --make works while simply ghc does not?
BTW, my ghc's version is
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.10.5

Answer 1

From the GHC docs:
--make
In this mode, GHC will build a multi-module Haskell program automatically, figuring out dependencies for itself. If you have a
straightforward Haskell program, this is likely to be much easier, and
faster, than using make. [..]
This mode is the default if there are any Haskell source files mentioned on the command line, and in this case the --make option can
be omitted.
Hence your line
ghc -o main Main.hs Lib.o
actually means
ghc -o main --make Main.hs Lib.o
which will compile and link Main.hs and all its dependencies (including Lib.o), adding another Lib.o during linking. This will link Lib.o twice, triggering the linker error reported by the OP.
I guess this was changed in GHC after Real World Haskell was written.