error in your article? about meaning of safe/unsafe in "foreign import"

Duncan Coutts duncan.coutts at
Fri May 20 06:28:21 EDT 2005

On Fri, 2005-05-20 at 11:30 +0200, Peter Simons wrote:

> Since pure FFI calls don't have any side-effects, they are
> always safe to be called unsafely. (Yes, the choice of the
> words "safe" and "unsafe" is a bit unfortunate in the standard
> here.)

To try and undo this confusion we need to recall what the safe/unsafe
are referring to. This safe/unsafe tag is used to describe both a
property of the form of the call (what degree of precaution we take in
making the call) and also a property of the thing we are calling
(whether it has side effects or can trigger callbacks into the Haskell

The two uses are basically opposite:

For a C procedure that can trigger callbacks, we have to take extra
procautions when calling it, that is we have to make the call in a
"safe" way.

For a C procedure that cannot trigger callbacks, we don't have to worry
at all when calling it, that is we can omit various cleanups, locks and
checks thus doing the call in an "unsafe" way.

So to sumarise the pairings:
      * you _must_ make a safe call to an unsafe foreign function
      * you _may_ make an unsafe call to a safe foreign function

It's a contravariance :-)


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