unsafeCoerce# between integral and floating point types
daniel.is.fischer at web.de
Thu Jul 8 10:49:00 EDT 2010
The docs for unsafeCoerce# say:
"The following uses of unsafeCoerce# are supposed to work (i.e. not lead to
spurious compile-time or run-time crashes):
# Casting any lifted type to Any
# Casting Any back to the real type
# Casting an unboxed type to another unboxed type of the same size (but not
coercions between floating-point and integral types)
My experience so far is consistent with the assumption that e.g.
unsafeCoerce# :: Word64 -> Double
is like a cast from (uint64_t *) to (double *), i.e. a bit-pattern-
preserving transformation (although by the docs, that use is undefined).
Would that assumption generally hold for
unsafeCoerce :: a -> b
where a and b are single constructor data types wrapping unboxed types of
the same bit-size and hence the use of unsafeCoerce# between such types
would produce reliable results *on the same machine with the same OS (and
GHC version?)* ?
And what about e.g.
unsafeCoerce# :: Word64# -> Double# ?
By the docs, that isn't supposed to work. Is it not supposed to work only
because it's not value-preserving (unsafeCoerce# 1## /=## 1.0##) or are
there more pitfalls?
If there are more pitfalls, is there any chance of getting a function which
reinterprets the bit-patterns?
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