Debugging tips and tricks

Use printLn(toString(...)) to query variables on the command line and in the etch playground.


In a live environment, the etch compiler will strip out printLn() statements.


Our compiler is very strict. It will complain about:

  • Unknown and unsupported symbols, operators, and indexes.
  • Lack of types for variables and parameters.
  • Assigning null to primitive types.
  • Incorrect, incompatible, and uninferrable types in statements and functions.
  • Duplicate variables, parameters, and functions.
  • Incomparable types in boolean tests.
  • Unmatched and unexpected beginnings, middles, and endings in selections, iterations, statements, and blocks.
  • Incorrectly formed statements.
  • Incorrect amount, or lack of, operands in boolean tests.
  • Break and continue statements outside while or for loops.
  • Non-functions, unmatched functions, and functions that do not return a value.
  • Local function definitions.
  • Returning null or no value from functions.
  • Attempts to construct primitives and unmatched constructors.
  • Mismatches between type and instance functions - awaiting clarification.
  • Member access support issues with dot operator - awaiting clarification.
  • while, for, if, var, return, break, continue, variables, and expressions declared at topmost scope.

System arguments

System.Argc() and System.Argv() give us access to compiler argument count and values. In the following code, first we print the number of compiler arguments, then we list them.

function main()
  var message : String;

  printLn("System args: " + toString(System.Argc()));

  // print the argument values
  for(i in 0:System.Argc() - 1)
    message = toString(i) + " = " + System.Argv(i);

Running the code from the command line with zero flags -- sends arguments to the vm-lang executable.

For example, run the code snippet above with zero flags -- and an example argument hello like this:

./vm-lang sysargs-examples.etch -- hello

You should see the following output:

 F E      vm-lang v0.1.0-30-g557389e0
   T C     Copyright 2018-2019 (c) Fetch AI Ltd.

System args: 2
0 = ./vm-lang
1 = hello

etch features like this are useful for playing around with and for debugging in a dev environment. They should not be included in smart contract code.

For smart contracts, there are more flags, such as -data, which do specific things. We explain coding, compiling, contract flags, and all about etch smart contracts here.