blob: 11a940b34d9d07cc8ad1285140e60934a2e4328e (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
|
.. _assertions:
Assertions
==========
.. index::
single: Assertions
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 :base-ref:`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.
.. index::
pair: assertions; disabling
GHC ignores assertions when optimisation is turned on with the
:ghc-flag:`-O` flag. That is, expressions of the form ``assert pred e``
will be rewritten to ``e``. You can also disable assertions using the
:ghc-flag:`-fignore-asserts` option. The option
:ghc-flag:`-fno-ignore-asserts <-fignore-asserts>`
allows enabling assertions even when optimisation is turned on.
Assertion failures can be caught, see the documentation for the
:base-ref:`Control.Exception.` library for the details.
|
