loadConcr, unloadConcr and addLiteral modify the Concr structure as a side
effect. This means that other functions with a Concr argument (e.g. parse
and linearize) are no longer pure.
Possible solutions:
1. Don't try to hide the imperative nature of the C run-time system: remove
all uses of unsafePerformIO and let all functions operate in the IO monad.
2. Don't export functions with side effects. Perhaps the desired functionality
of loadConcr, unloadConcr and addLiteral can be folded into readPGF.
The Concr structures can then treaded as immutable after after the
readPGF function returns...
PGF exports the public, stable API.
PGF.Internal exports additional things needed in the GF compiler & shell,
including the nonstardard version of Data.Binary.
From the documentation: the throwIO variant should be used in preference to
throw to raise an exception within the IO monad because it guarantees ordering
with respect to other IO operations, whereas throw does not.
Also removed some unused imports.
