6.19.4. AssertionsΒΆ
If you want to make use of assertions in your standard Haskell code, you could define a function like the following:
assert :: Bool -> a -> a
assert False x = error "assertion failed!"
assert _ x = x
which works, but gives you back a less than useful error message – an assertion failed, but which and where?
One way out is to define an extended assert
function which also
takes a descriptive string to include in the error message and perhaps
combine this with the use of a pre-processor which inserts the source
location where assert
was used.
GHC offers a helping hand here, doing all of this for you. For every use
of assert
in the user’s source:
kelvinToC :: Double -> Double
kelvinToC k = assert (k >= 0.0) (k-273.15)
GHC will rewrite this to also include the source location where the assertion was made,
assert pred val ==> assertError "Main.hs|15" pred val
The rewrite is only performed by the compiler when it spots applications
of Control.Exception.assert
, so you can still define and use your
own versions of assert
, should you so wish. If not, import
Control.Exception
to make use assert
in your code.
GHC ignores assertions when optimisation is turned on with the
-O
flag. That is, expressions of the form assert pred e
will be rewritten to e
. You can also disable assertions using the
-fignore-asserts
option. The option
-fno-ignore-asserts
allows enabling assertions even when optimisation is turned on.
Assertion failures can be caught, see the documentation for the Control.Exception library for the details.