Command Line Options

Leon’s command line options have the form --option or --option=value. To enable a flag option, use --option=true or on or yes, or just --option. To disable a flag option, use --option=false or off or no.

Additionally, if you need to pass options to the scalac frontend of Leon, you can do it by using a single dash -. For example, try -Ybrowse:typer.

The rest of this section presents command-line options that Leon recognizes. For a short (but always up-to-date) summary, you can also invoke leon --help.

Choosing which Leon feature to use

The first group of options determine which feature of Leon will be used. These options are mutually exclusive (except when noted). By default, --verify is chosen.

  • --eval

    Evaluates parameterless functions and value definitions.

  • --verify

    Proves or disproves function contracts, as explained in the Verification section.

  • --repair

    Runs program repair.

  • --synthesis

    Partially synthesizes choose() constructs (see Synthesis section).

  • --termination

    Runs termination analysis. Can be used along --verify.

  • --inferInv

    Infer invariants from the (instrumented) code (using Orb).

  • --instrument

    Instrument the code for inferring time/depth/stack bounds (using Orb).

  • --genc

    Translate a Scala program into C99 equivalent code (see Safe C Code section); requires --xlang.

  • --noop

    Runs the program through the extraction and preprocessing phases, then outputs it in the specified directory. Used mostly for debugging purposes.

  • --help

    Prints a helpful message, then exits.

Additional top-level options

These options are available to all Leon components:

  • --debug=d1,d2,...

    Enables printing detailed messages for the components d1,d2,... . Available components are:

    • datagen (Data generators)
    • eval (Evaluators)
    • genc (C code generation)
    • isabelle (The Isabelle-based solver)
    • leon (The top-level component)
    • options (Options parsed by Leon)
    • positions (When printing, attach positions to trees)
    • repair (Program repair)
    • solver (SMT solvers and their wrappers)
    • synthesis (Program synthesis)
    • termination (Termination analysis)
    • timers (Timers, timer pools)
    • trees (Manipulation of trees)
    • types (When printing, attach types to expressions)
    • verification (Verification)
    • xlang (Transformation of XLang into Pure Scala programs)
  • --functions=f1,f2,...

    Only consider functions f1, f2, ... . This applies to all functionalities where Leon manipulates the input in a per-function basis.

    Leon will match against suffixes of qualified names. For instance: --functions=List.size will match the method leon.collection.List.size, while --functions=size will match all methods and functions named size. This option supports _ as wildcard: --functions=List._ will match all List methods.

  • --solvers=s1,s2,...

    Use solvers s1, s2,... . If more than one solver is chosen, all chosen solvers will be used in parallel, and the best result will be presented. By default, the fairz3 solver is picked.

    Some solvers are specialized in proving verification conditions and will have hard time finding a counterexample in case of an invalid verification condition, whereas some are specialized in finding counterexamples, and some provide a compromise between the two. Also, some solvers do not as of now support higher-order functions.

    Available solvers include:

    • enum

      Uses enumeration-based techniques to discover counterexamples. This solver does not actually invoke an SMT solver, and operates entirely on the level of Leon trees.

    • fairz3

      Native Z3 with z3-templates for unfolding recursive functions (default).

    • smt-cvc4

      CVC4 through SMT-LIB. An algorithm within Leon takes up the unfolding of recursive functions, handling of lambdas etc. To use this or any of the following CVC4-based solvers, you need to have the cvc4 executable in your system path (the latest unstable version is recommended).

    • smt-cvc4-cex

      CVC4 through SMT-LIB, in-solver finite-model-finding, for counter-examples only. Recursive functions are not unrolled, but encoded through the define-funs-rec construct available in the new SMTLIB-2.5 standard. Currently, this solver does not handle higher-order functions.

    • smt-cvc4-proof

      CVC4 through SMT-LIB, for proofs only. Functions are encoded as in smt-cvc4-cex. Currently, this solver does not handle higher-order functions.

    • smt-z3

      Z3 through SMT-LIB. To use this or the next solver, you need to have the z3 executable in your program path (the latest stable version is recommended). Inductive reasoning happens on the Leon side (similarly to smt-cvc4).

    • smt-z3-q

      Z3 through SMT-LIB, but (recursive) functions are not unrolled and are instead encoded with universal quantification. For example, def foo(x:A) = e would be encoded by asserting

      \[\forall (x:A). foo(x) = e\]

      even if e contains an invocation to foo.

      Currently, this solver does not handle higher-order functions.

    • unrollz3

      Native Z3, but inductive reasoning happens within Leon (similarly to smt-z3).

    • ground

      Only solves ground verification conditions (without variables) by evaluating them.

    • isabelle

      Solve verification conditions via Isabelle.

  • --strict

    Terminate Leon after each phase if a non-fatal error is encountered (such as a failed verification condition). By default, this option is activated.

  • --timeout=t

    Set a timeout for each attempt to prove one verification condition/ repair one function (in sec.)

  • --xlang

    Support for additional language constructs described in XLang. These constructs are desugared into Pure Scala before other operations, except for the --genc option which uses the original constructs to generate Safe C Code.

Additional Options (by component)

The following options relate to specific components in Leon. Bear in mind that related components might still use these options, e.g. repair, which invokes synthesis and verification, will also use synthesis options and verification options.


  • --parallel

    Check verification conditions in parallel.

File Output

  • --o=dir

    Output files to the directory dir (default: leon.out). Used when --noop is selected.

    When used with --genc this option designates the output file.

Code Extraction

  • --strictCompilation

    Do not try to recover after an error in compilation and exit Leon.


  • --cegis:opttimeout

    Consider a time-out of CE-search as untrusted solution.

  • --cegis:shrink

    Shrink non-deterministic programs when tests pruning works well.

  • --cegis:vanuatoo

    Generate inputs using new korat-style generator.

  • --costmodel=cm

    Use a specific cost model for this search. Available: Naive, WeightedBranches

  • --derivtrees

    Generate a derivation tree for every synthesized function. The trees will be output in *.dot files.

  • --manual=cmd

    Override Leon’s automated search through the space of programs during synthesis. Instead, the user can navigate the program space manually by choosing which deductive synthesis rules is instantiated each time.

    The optional cmd argument is a series of natural numbers in the form n1,n1,...,nk. It represents the series of command indexes that the search should instantiate at the beginning of the search. Useful for repeated search attempts.

Fair-z3 Solver

  • --checkmodels

    Double-check counter-examples with evaluator.

  • --codegen

    Use compiled evaluator instead of interpreter.

  • --evalground

    Use evaluator on functions applied to ground arguments.

  • --feelinglucky

    Use evaluator to find counter-examples early.

  • --unrollcores

    Use unsat-cores to drive unrolling while remaining fair.

CVC4 Solver

  • --solver:cvc4=<cvc4-opt>

    Pass extra command-line arguments to CVC4.


  • --isabelle:dump=<path>

    Makes the system write theory files containing the translated definitions and scripts. The generated files may be loaded directly into Isabelle, but are not guaranteed to work, as pretty-printing Isabelle terms is only an approximation.

  • --isabelle:mapping

    Controls function and type mapping. On by default. When switched off, neither functions nor types are mapped at all.

  • --isabelle:strict

    Strict prover mode. On by default. Replays all referenced proofs from the library when verifiying a Leon source file. Keeping it enabled prevents unsound proofs when postconditions or mappings in the library are wrong. When disabled, a warning is printed.

Invariant and Resource Bound Inference

These options are to be used in conjunction with --inferInv.

  • --minbounds=lowerlimit

    Minimize the inferred coefficients based on the rate of growth of the corresponding term in the bound. Coefficients of faster growing terms have higher priority than coefficients of smaller growing terms. lowerlimit is a (possibly negative or zero) integer that specifies a lower limit up to which the minimization can proceed. A lower limit is mandatory.

  • --timeout=s

    A overall timeout in seconds for the inference phase. The tool will exit after s seconds

  • --solvers=sol

    Use the SMT solver sol for checking verification conditions. sol could be either orb-smt-z3 or orb-smt-cvc4. orb-smt-z3 is generally faster than orb-smt-cvc4. But, orb-smt-cvc4 works better for theory of sets, and datatypes .

  • --benchmark

    Dump useful statistics about the performance of inference to file.

  • --assumepreInf

    Assume preconditions of callees while unfolding callees during inference

  • --disableInfer

    Disable automatic inference of auxiliary invariants and only infer values for holes

  • --nlTimeout=s

    A timeout in seconds for nonlinear solving step, which is by default 15s

  • --vcTimeout=s

    A timeout in seconds for solving verification conditions, which is by default 15s