How safe is `unsafePerformIO (newTVarIO 0)`?

Question

I've noticed this idiom in Data.Unique:

uniqSource :: TVar Integer
uniqSource = unsafePerformIO (newTVarIO 0)
{-# NOINLINE uniqSource #-}

Is it guaranteed to only run once?

Answer 1

In GHC, yes.¹ See the documentation for more information; there is a variant unsafeDupablePerformIO that can be executed multiple times that avoids the overhead dedicated to achieving this guarantee.

Note that unsafePerformIO to create mutable variables isn't safe in general; as described in the documentation, you can create a polymorphic reference and use it to implement unsafeCoerce. That's not something you're likely to do accidentally, though, and it doesn't apply to the code in question (since the type of the reference is specified explicitly).

The safe-globals package abstracts this "idiom" (while useful in some cases, it's generally considered an antipattern, and should not be used in normal code) in a way that ensures safety.

See also my previous answer on unsafePerformIO and the caution that must be used when applying it.

¹ I'm pretty sure it applies to all other implementations, too; the special care GHC takes to avoid repeated execution is only necessary in a threaded setting, and I don't know of any other threaded implementations of Haskell. GHC is the only implementation people really use these days, though...