2.11. FAQ and Things To Watch Out For

The interpreter can't load modules with foreign export declarations!

Unfortunately not. We haven't implemented it yet. Please compile any offending modules by hand before loading them into GHCi.

-O doesn't work with GHCi!

For technical reasons, the bytecode compiler doesn't interact well with one of the optimisation passes, so we have disabled optimisation when using the interpreter. This isn't a great loss: you'll get a much bigger win by compiling the bits of your code that need to go fast, rather than interpreting them with optimisation turned on.

Unboxed tuples don't work with GHCi

That's right. You can always compile a module that uses unboxed tuples and load it into GHCi, however. (Incidentally the previous point, namely that -O is incompatible with GHCi, is because the bytecode compiler can't deal with unboxed tuples).

Concurrent threads don't carry on running when GHCi is waiting for input.

This should work, as long as your GHCi was built with the -threaded switch, which is the default. Consult whoever supplied your GHCi installation.

After using getContents, I can't use stdin again until I do :load or :reload.

This is the defined behaviour of getContents: it puts the stdin Handle in a state known as semi-closed, wherein any further I/O operations on it are forbidden. Because I/O state is retained between computations, the semi-closed state persists until the next :load or :reload command.

You can make stdin reset itself after every evaluation by giving GHCi the command :set +r. This works because stdin is just a top-level expression that can be reverted to its unevaluated state in the same way as any other top-level expression (CAF).

I can't use Control-C to interrupt computations in GHCi on Windows.

See Section 11.2, “Running GHCi on Windows”.

The default buffering mode is different in GHCi to GHC.

In GHC, the stdout handle is line-buffered by default. However, in GHCi we turn off the buffering on stdout, because this is normally what you want in an interpreter: output appears as it is generated.