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Java Input-Output Monad
There is a newer version: 3.0.0-RC2
package jio.console;

import jio.IO;
import jsonvalues.JsObj;

import java.util.Arrays;
import java.util.function.Function;

class SetVarCommand extends Command {

  private static final String COMMAND_NAME = "var-set";

  public SetVarCommand() {
    super(COMMAND_NAME,
          """
              Stores the specified value into the the specified variable.
              set var value
              Examples:
                  $command age 40
                  $command counter $var""".replace("$command",
                                                   COMMAND_NAME)
         );
  }

  private static IO setVarValue(State state,
                                        String varName,
                                        String newValue
                                       ) {

    return IO.lazy(() -> {
                     String oldValue = state.variables.get(varName);
                     state.variables.put(varName,
                                         newValue);
                     return String.format("%s from %s to %s",
                                          varName,
                                          oldValue,
                                          newValue
                                         );
                   }
                  );
  }

  @Override
  public Function> apply(final JsObj conf,
                                              final State state
                                             ) {
    return tokens -> {
      int nTokens = tokens.length;

      if (nTokens == 1) {
        return Programs.ASK_FOR_PAIR(new Programs.AskForInputParams("Type the name of the variable"),
                                     new Programs.AskForInputParams("Type the value")
                                    )
                       .then(pair -> setVarValue(state,
                                                 pair.first(),
                                                 pair.second()));
      }

      if (nTokens == 2) {
        return Programs
            .ASK_FOR_INPUT(new Programs.AskForInputParams("Type the value"))
            .then(value -> setVarValue(state,
                                       tokens[1],
                                       value));
      }

      return setVarValue(state,
                         tokens[1],
                         String.join(" ",
                                     Arrays.stream(tokens)
                                           .toList()
                                           .subList(2,
                                                    tokens.length))
                        );


    };
  }
}