Unsafe functions
Unsafe definitions
[Using the FFI] you can import any C function with a pure type, which also allows you to wreak arbitrary havoc. We enable the user to disguise arbitrary machine code as a Haskell function of essentially arbitrary type. In comparison,
unsafePerformIO
seems angelic.
Definitions, not functions!
Regarding an extra primitive definition val execute: α Beh ⟶ α
for PFL+:
Unfortunately
execute
is not a safe extension to a functional language because it is not unfoldable—witness the expression(x ⟼ x-x)(execute(i?x. Ret x))which unfolds to
execute(i?x. Ret x) - execute(i?x. Ret x)The first executes by reading one value from channel
i
, then returning0
; while the second reads two values fromi
, and returns their difference. The first makes one suspension, the second two.Nor is
execute
a function; the subexpressionexecute(i?x. Ret x)
can take on two different values in the expression above.
- PFL+: A Kernel Scheme for Functional I/O (pages 15-16 of 28).
A rogues' gallery
These are just some of the various unsafe definitions lurking in libraries:
unsafeLocalState :: IO a -> a
unsafePerformIO :: IO a -> a
inlinePerformIO :: IO a -> a
unsafeInterleaveIO :: IO a -> IO a
unsafeInterleaveST :: ST s a -> ST s a
unsafeIOToST :: IO a -> ST s a
unsafeIOToSTM :: IO a -> STM a
unsafeFreeze
,unsafeThaw
unsafeCoerce# :: a -> b
Unsafe definitions can:
- break type safety (
unsafeCoerce#
,unsafeLocalState
,unsafePerformIO
),
- or break equational reasoning (
unsafeInterleaveIO
).
Their use would require some kind of assurance on the part of the programmer that what they're doing is safe.
unsafe
is also a keyword which can be used in a foreign import declaration.
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