[Haskell-cafe] [C Binding] Turning the mutable to immutable?

Edward Z. Yang ezyang at MIT.EDU
Thu Jul 8 00:24:24 EDT 2010

To answer the question in your subject, “Very Carefully.”

While I don’t know much about your particular problem domain (and it seems
others have given useful advice), I can say some general things about
making mutable things immutable.

There is a very simple way to make something mutable immutable: say that you
won’t ever mutate it again!  Doing this correctly is two-fold: you first have
to know when you can do this and when it is useful (which requires in-depth
understanding of the side-effects that the low-level API invokes), and second
is knowing how to encapsulate your interface so that there is no type-checking
use-case of your code (without unsafePerformIO) that accidentally mutates a
pure structure.

You can use this to do a standard pattern seen in ST and others: mutate during
creation inside a monad, but when the monad is done running, return a pure
version of the output with unsafePerformIO.  If you can guarantee that another
caching the pointer so that if someone else calls your function with the
same arguments, you return the same pointer, is safe, then this is ok.

Memcopying a datastructure when you need to modify it, while a cringe-worthy
offense in your theoretical CS class, is a surprisingly practical and not-to-bad
performing technique for making your data persistent.  It works better the smaller
the data structure is, and you'd be surprised how many C libraries implement
some feature with a memory copy (moving GC, anyone?)


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