>
> > > - Great care should be exercised in the use of this function. Not only
> > > - because of the danger of introducing side effects, but also because
> > > - \code{unsafePerformIO} may compromise typing, for example, when it is used
> > > - in conjunction with polymorphic references.
> >
> > Or maybe it would be better to provide some useful guidance? How about,
> >
> > To preserve the soundness of the type system, the result of
> > unsafePerformIO should always have a monomorphic type. For
> > example,
> >
> > listRef = unsafePerformIO (newIORef [])
> >
> > is unsafe, while
> >
> > listRef = unsafePerformIO (newIORef ([] :: [Int]))
> >
> > is type safe. In the first case listRef is assigned type IORef
> > [a], which makes it possible to store a list of one type and fetch
> > it with a different type.
>
> Unfortunately, this example is not directly applicable. As
> Ross pointed out, it is already ruled out by the determinism
> requirement. Moreover, `IORef's are neither part of H98 nor
> of the FFI. The construction of a corresponding example
> with `Ptr' that uses `unsafePerformIO' deterministically is
> possible, but IMHO a bit to verbose for inclusion at this
> point. However, I have changed the above cited warning to
> read
>
> Great care should be exercised in the use of this function. Not only
> because of the danger of introducing side effects, but also because
> \code{unsafePerformIO} may compromise typing; in particular, the result of
> \code{unsafePerformIO} should always have a monomorphic type.
>
> This at least describes the typing problem more precisely.
>
> Cheers,
> Manuel
>
Manuel,
"should always have" is unfortunately ambiguous: does it mean "you should
ensure that...", or "we believe that..., but we're not completely sure". I
suggest changing the last phrase to
...; to avoid this, the programmer should ensure that the result of
unsafePerformIO has a monomorphic type.
John
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