Curry Package verify-non-fail

verify-non-fail: A tool to verify Curry operations as non-failing

This package contains a tool to verify that all operations in a given module are non-failing, i.e., their evaluation does not result in a failure, if they are called with appropriate arguments. The tool automatically infers abstract call types specifying supersets of appropriate arguments.

As an example, consider the following operation to compute the last element of a given list:

last :: [a] -> a
last [x]            = x
last (_ : xs@(_:_)) = last xs

In the default mode (where types are abstracted into a set of allowed top-level constructors), the inferred call type is

last: {:}

specifying that last does not fail if the argument is a non-empty list, i.e., evaluable to some data term rooted by the list constructor “:”.

When an operation with such a restricted call type is used, the tool checks whether it is used in a context which ensures its call type. For instance, the operations

head (x:xs) = x


tail (x:xs) = xs

have the inferred call types

head: {:}
tail: {:}

When these operations are applied, it should be ensured (in a top-level functional computation) that the actual arguments are non-empty lists, as in the following operation:

readCommand :: IO ()
readCommand = do
  putStr "Input a command:"
  s <- getLine
  let ws = words s
  case null ws of True  -> readCommand
                  False -> processCommand (head ws) (tail ws)

The tool automatically verifies that the operation readCommand is non-failing.

The ideas of the implementation of this tool are described in:

M. Hanus: Inferring Non-Failure Conditions for Declarative Programs, Proc. of the 17th International Symposium on Functional and Logic Programming (FLOPS 2024), Springer LNCS 14659, pp. 167-187, 2024

Arithmetic non-fail conditions

If the SMT solver Z3 is installed and the executable z3 can be found in the path, the tool can also infer and verify arithmetic non-fail conditions. For instance, it infers for the factorial function defined by

fac :: Int -> Int
fac n | n == 0 = 1
      | n > 0  = n * fac (n - 1)

the non-fail condition

fac'nonfail :: Int -> Bool
fac'nonfail v1 = (v1 == 0) || (v1 > 0)

specifying that the argument must be non-negative in order to ensure that fac does not fail. With this information, it also proves that the following I/O operation does not fail (where the operation readInt reads a line until it is an integer):

printFac :: IO ()
printFac = do
  putStr "Factorial computation for: "
  n <- readInt
  if n<0 then putStrLn "Negative number, try again" >> printFac
         else print (fac n)


If you downloaded the sources of this package, then the tool can be installed via the command

> cypm install

in the main directory of this package. Otherwise, the tool can also be downloaded and installed by the command

> cypm install verify-non-fail

This installs the executable curry-calltypes in the bin-directory of CPM.

If one uses KiCS2 to install this tool, one should use version 3.1.0 of April 11, 2024 (or newer) due to a memory leak in an older version of KiCS2.


In order to support a modular analysis of applications consisting of several modules, the tool caches already computed analysis results of modules in under the directory ~/.curry_verify_cache. This path is defined in Verify.Files.getVerifyCacheDirectory. To store the analysis results for different Curry systems and abstract domains separately, the analysis results for a Curry module which is stored in a file with path MODULEPATH.curry are contained in


For instance, the call types of the prelude for KiCS2 Version 3.1.0 w.r.t. the abstract domain Values are stored in


provided that KiCS2 is installed at /opt/kics2 so that the prelude is contained in the file /opt/kics2/lib/Prelude.curry.

All cache files (for the Curry system used to install this tool) can be deleted by the command

curry-calltypes --delete


Checkout with CPM:
cypm checkout verify-non-fail 1.2.0
Package source:
verify-non-fail-1.2.0.tar.gz [browse]
Source repository: