Note [Precise vs imprecise exceptions]

An exception is considered to be /precise/ when it is thrown by the 'raiseIO#'
primop. It follows that all other primops (such as 'raise#' or
division-by-zero) throw /imprecise/ exceptions. Note that the actual type of
the exception thrown doesn't have any impact!

GHC undertakes some effort not to apply an optimisation that would mask a
/precise/ exception with some other source of nontermination, such as genuine
divergence or an imprecise exception, so that the user can reliably
intercept the precise exception with a catch handler before and after
optimisations.

See also the wiki page on precise exceptions:
https://gitlab.haskell.org/ghc/ghc/wikis/exceptions/precise-exceptions
Section 5 of "Tackling the awkward squad" talks about semantic concerns.
Imprecise exceptions are actually more interesting than precise ones (which are
fairly standard) from the perspective of semantics. See the paper "A Semantics
for Imprecise Exceptions" for more details.