kscript from docker

This is one of those cases that you go down the rabbit hole and end up doing ten different things before the one that you initially wanted to!

My original intention was to try out kscript. What I ended up doing is learning about dockerfiles, building images and trying to run .kts files without having kscript, or kotlin, installed locally in my machine!

The result

A github repository that you can clone, run the install script and have a kscript executable that works as described here.


I wanted to play with kscript but did not want to install it on my machine. I don’t like bloating my OS by having libraries and software that might not be used often.


Having everything installed in a docker image and use containers to play with kscript was a one way street for what I wanted to achieve.


It was my first dockerfile but there are tons of tutorials that helped me out. I ended up using alpine, installing java through its repositories, sdkman using its installation guide and kotlin, kscript through sdkman!

Finally, kscript was added as the image’s entry point which means that every time a container is being run kscript will be the first command that gets executed.

And it worked for the most parts. By executing

docker run –rm -i kscript <params>

rm to remove the container after its done, -i to have the script’s output passed to the terminal

I was able to use kscript as described in its repository, except for the case of having a .kts file!


For the case of a .kts file what turned out to solve my problem was to pass the file’s entire content. And this is part of what kscript-router.sh does. It checks if the first parameter is a .kts file and if so it passes its contents to the container. In every other case it passes all given parameters directly to the container.


To tie everything together I added a script that creates the image, adds the router script to the user’s path and creates an alias, named kscript, for executing the router.


Readable codebase: extract conditions / predicates

A quick way to make a codebase more readable is to extract long or complex conditions / predicates to their own functions. The function’s name will explain to the reader what is being checked allowing her to move forward instead of trying to figure out the context or the calculations that are taking place.

The task

Completed tasks of the last two days

Lets assume that we have a piece of code that prints all tasks that were created in the last two days and are, as the business experts calls them, completed. Which means that are either Resolved or Cancelled.

We have two distinct concepts that can be extracted. One that will explain the date calculations and one that will force the business knowledge behind those two statuses.


The calculations can be extracted to a private function but since we are using Kotlin we will extract them to an extension function to make the code even more readable:


Another benefit of having “the mentality of extraction” (yep, I just made it up) is that eventually we will come across a case like the completed which is a business term that has not been transferred to the codebase!

So we don’t just make the code more readable but we improve our domain representation and force a rule that until now was known through documentations or, even worse, through conversations only.

The reader will not have to make any calculations while reading this code or trying to understand why only those statuses are being checked.

The code speaks for itself!

Write comments only when they answer the “Why?”

This is the one and only question that I ask myself when I see comments or when I feel that I need to write some:

Do these comments provide the reason why this code was written?

If not then 99.9% we don’t need them.

“Remove right margin”

I recently came across a block of code that looked like this:

In this example the comment simply describes what the code does. I am pretty sure that everyone can figure that out but only the code’s author knows why we need to remove the margin under those circumstances. I am also sure that in six months (in my case in six days) even the author won’t remember the reason behind this code.

Extract if’s logic

To be fair I do understand the author’s urge to write this comment. Those unavoidable negations, because of the system’s/ui’s APIs, look like they need to be clarified.

Fortunately there is a better way to do that: we extract if’s logic to its own method and name it accordingly:

Extract if’s body

We can go a step further and extract if’s body to its own method too, having the code replacing the comment completely:

Answer the “Why?”

So now what is missing is the reason behind the margin’s removal. A comment that answers the why and completes the reader’s understanding about the code she/he reads.

“I can see what is happening and I know why too!”

Code review: don’t just request a change

When reviewing a PR I try to elaborate on my proposals for two reasons:

1. Out of respect to my colleague

Every piece of code is part of an effort that took time and thought. Requesting a code change by simply asking it (i don't like it, change it) or by dictating it (do it this way) demotes all the work that has been done.

I figured that, since my colleague spent a few hours to come up with a solution I owe her/him more than a few seconds!

2. I solidify my knowledge or even better, learn something new

By trying to justify the reason behind a request I end up understanding better why I prefer a solution or a format over another.

When I provide an example or when I do a mini investigation to help me clarify why something must change I often find that (a) many things that I thought I knew were not as I had them in mind and (b) I now understand a concept much better because I had to write about it. You don’t master something if you can’t explain it!

PS: never forget that it might say “request a change” but in reality you start a conversation between two professionals that will eventually benefit of the project

Cherry pick all the commits from a merged branch

I recently had to revert an entire feature and create a new branch that would be merged in the future. In git terms, I had to revert my merged branch, create a new one and cherry pick the reverted commits to it.

I’ve been working with git for years but up until now I never had to revert a commit, let alone an entire branch. Turns out reverting is quite simple and can be done in a single command but cherry-picking all the branch’s commits is a repeatable and tedious task since we need to copy each commit, create a string out of them and give it to cherry-pick.

What I wanted was a way to give git the merge commit and let it figure out which commits to cherry-pick. For example, in the case described below:

I would like to cherry-pick all five commits edaa436, 5d48f23, a8f2b4d, 63f2f4e, 1dc3f6a by just providing to git the commit f51c08c.

I had already done a “custom” git-alias so I figured I could do something similar and create cherry-grab: git cherry-grab f51c08c


Two things helped in creating cherry-grab and the first one is the ^ suffix that one of its usages is to get the commit’s parent. So if we run git show edaa436^ git will translate it to git show 84f8f45.

In our case where the commit is the result of a merge we can specify which of the two parents we want. f51c08c^1 will be translated to 84f8f45 and f51c08c^2 to 1dc3f6a (in a similar way if we need the nth parent we request it by using ^n).

git rev-list <commit>

The second thing was rev-list which returns a list with all the commits that can be reached through the parent “pointer” starting from the provided commit. So if we run git rev-list edaa436 git will return edaa436, 84f8f45 and 55da68d.

Also, rev-list can exclude all commits that are reachable from a given commit as long as we prefix it with the caret (^) symbol. So if we run git rev-list edaa436 ^84f8f45 git will return just edaa436.


First we need to get the list with all of the branch’s commits. To do that we will use the rev-list command and request all commits starting from the merge-commit’s second parent but exclude all commits that are reachable from the merge-commit’s first parent:

git rev-list 1dc3f6a ^84f8f45

this returns 1dc3f6a, 63f2f4e, a8f2b4d, 5d48f23, edaa436 which are the commits that construct the merged branch.

Then we need to reverse this list so that we can apply each commit in the proper order:

git rev-list 1dc3f6a ^84f8f45 | tac

which returns edaa436, 5d48f23, a8f2b4d, 63f2f4e, 1dc3f6a.

Having the commits in the correct order we just need to pass them to the cherry-pick command to apply their changes:

git rev-list 1dc3f6a ^84f8f45 | tac | xargs git cherry-pick

Finally we need to put in an alias to make it reusable:

Nothing fancy or special, we just wrap the sequence of command with a function and tell git to call that function when ever we use the cherry-grab alias.

In order to make it reusable we replace the parent commits with $1^2 and $1^1 respectively which are translated to the second and first parent of the passed commit.

That’s it:

Git alias with parameters

In the company I work we use the gitflow workflow and in every feature branch we make sure that part of its name is the task’s id which includes a unique number. So what we end up with is something like:


What I wanted was an easy way to checkout a branch using just the number. I wanted (spoiler: and succeeded) something like this:

git ch 12345


Lets see how we can checkout a branch without an alias but by using the task’s number.

Get the branch

We can get the branch by listing all branches (git branch) and then grep for the one that contains the task’s number:

git branch | grep 12345

this will return feature/T12345-make-the-app-rock

Checkout to it

Having a way to get the branch makes the checkout quite easy since we can use bash’s command substitution to execute the above commands and use the result directly in the checkout:

git checkout $(git branch | grep 12345)

Put it in a function

We are now able to checkout the task’s branch but it would be nice if we could change the task’s number more easily. Lets create a function and pass the number as a parameter:

  • The $1 is where the parameter will be used (if we had more parameters we would use $2 for the second, $3 for the third etc).
  • You can check the function by writing it directly in the terminal. Just make sure that you add newlines (press enter):

The alias

We can change branches easily but once we close the current terminal session we lose everything. Our goal is to have all that in a git alias.

Creating an alias can be achieved either by using git config

git config –global alias.ch checkout

or by doing it manually and adding the alias directly into the .gitconfig file (usually located in the home directory). The file should end up with something like:

Having just the alias will not help us much. As is, it will work only if we give it the entire branch:

git ch feature/T12345-make-the-app-rock

What we need is a way to connect the function that we created with the alias.

The answer is provided by git itself since it has a way to create aliases that do not execute a git command but an external command:

However, maybe you want to run an external command, rather than a Git subcommand. In that case, you start the command with a ! character. This is useful if you write your own tools that work with a Git repository.

git docs

Following the docs we can change the gitconfig file to look like this (this time the change must be made only manually because our function will need to have newlines):

What will happen when we write git ch 12345 is:

  1. git ch will be replaced with everything inside the "..." (minus the !) ending up with two commands
  2. the first one creates the f function and
  3. the second one calls f with the parameter 12345: f 12345

And that’s it! We now have a git alias that will allow us to checkout a branch using part of its name.

feature image by Zach Reiner @ unsplash

A smooth refactor using sealed classes and a factory function

The problem

Lets say we have a contacts app and one of the screens shows the contact’s phone number.

The problem with that code is that we can easily end up with instances that contain invalid state:

A phone number screen with an empty phone number!

Approach #1:

One way to prevent it is to add some logic in the presentation layer:

Open the phone number screen only if the phone number is not empty

Unfortunately this approach does not provide an actual solution but a patch. Our main goal is to have a PhoneNumberScreen that handles ONLY valid phone numbers.

Approach #2:

What we need is to move the necessary checks inside the PhoneNumberScreen class.

We could check the number’s validity on render and show some kind of message when there is no phone number.

It is quite clear that this solution provides a bad UX. Why open a screen when the user cannot use it? Also, in any additional usage of phoneNumber inside the PhoneNumberScreen we need to make the same check as in render() and handle both of its states.

Approach #3:

What we really need is to make sure that if the screen is created, then it is certain that it has a valid phone number. There are two ways to achieve that. The first one is by checking upon creation that the phone number is valid and throw an exception if it is not.

It works but we need to document it and add a try-catch wherever we create a screen instance.

The second one is by having a helper function that creates the screen only if the phone number is valid.

This also works as expected but once again we need to document it and on top of that handle any null values returned by the helper function.

Nevertheless this solution seems fine and for a very small code base is quite acceptable. The problem is that it does not scale alongside the code base. Imagine how many helper functions we need to implement every time we have to use a phone number instance in our components if we want to keep them “clean”.

The actual problem

The actual problem lies in the PhoneNumber itself. It represents more than one states and each time we use an instance of it we must “dive” in its value and translate it to that state.

What we really need is a better representation of a valid and invalid phone number.

Final approach

This is where we use sealed classes and separate the two states:

This way we accomplish our main goal: we change the PhoneNumberScreen to accept only instances of ValidPhoneNumber and can now be certain that the screen will be used only with valid data. The code is self documented and any further development in the screen class will not have to consider other states for the phone number:

One big drawback of this change is that every usage of the PhoneNumber class has just broke (see: creation of Contact instances).

Fortunately there is a quick solution for that! Factory function:

A function that has the same name with the previously used class (PhoneNumber) and takes a single string parameter. If the parameter is not empty it returns a ValidPhoneNumber. In any other case it returns an InvalidPhoneNumber:

The end result is almost the same as the starting point but this time we have clear states and components that can guarantee that they will not crash because of erroneous data:

A function’s intent should be revealed by its name

And a good way to know if it doesn’t is to read it where it gets used. If, after reading it, you have questions then something is wrong with the name.

A good example is this code that I run into:

if (handleClickItem(customer)) {

When I read this piece of code the very first thing that popped into my head was

How does the click gets handled?!

To figure that out I had to step into the function’s body and start reading it in order to understand what it does and when. It broke my flow and made me change context.

Turned out that this particular function, depending on the customer’s type, opens the contact’s screen and returns true or simply returns false. Having read something like:

if (openContactsScreenWhenCustomerIsCompany(customer)) {

would have been enough for me to simply keep reading and never look back.

Make your code reveal its usage

A small and simple example that shows one of the benefits of having domain objects.

Lets assume we need to make a request for some kind of a token and the flow for doing so goes like this:

  1. Validate an email address
  2. With the validated address validate a password
  3. With both the validated values request for the token

The not so revealing way

Q: Can the consumer of this API, without knowing the aforementioned flow, figure it out just by looking at the functions’ signatures?

A: I guess she could if the parameters’ names were like that. If not she needs to read the bodies of those functions to see that there is some kind of order that needs to be followed.

Q: Can the creator of this code be 100% certain that the functions will be used as expected and the passed parameters will be valid? For example, will requestToken always be called last and with valid addresses and passwords?

A: No! There is nothing that can guarantee that so, just to be safe, we check in every function that the provided values are valid and make the code flexible enough that, for example, each function could be used on its own. That will, potentially, lead to code duplication or unnecessary abstractions.

Q: What about errors and invalid values? Can the consumer of this API predict the code’s behavior on erroneous inputs without reading a documentation?

A: No. Just no.

The revealing way

First lets enrich our API with some, much needed, domain objects:

Having those constructs we can change the functions and make them reveal both the order they can be used and their behavior:

So lets pretend that we are the consumer of this API and all we have is the code.

Our main goal is to request for a token. By just looking at the functions’ signatures we see that there is a requestToken. Nice! Our task is half way done! What do we need for calling this function? A valid email address and a valid password. Ok. How do we get one of each?

Looking again at the signatures we see validateEmailAddress and validatePassword that return an EmailAddress and Password respectively. Lets check what those are. These are abstractions and each of them has been extended to a valid and an invalid state. The invalid one carries the error that occurred too! So, back to the functions.

We see that validating a password needs to be fed with a valid email address so we first need to call validateEmailAddress, then validatePassword and finally requestToken.

That’s it. We didn’t read the code of the functions and we didn’t have to worry about our inputs.

Use sealed classes for better domain representation

Lets start with the business logic.

We have a task. A task can be unassigned OR it can be assigned either to a user OR a group.

First implementation: the ugly way

One way to implement this is by putting all the logic in the task:

By default the task is unassigned and if we want an assigned task we provide a user or a group. The or factor is being enforced by a check in the constructor.

This implementation not only relies on nulls to represent the business logic but it also hides the logic from the developer who has to read the code to understand how to create an assigned task.

The null checking comes also up when we want to figure out if and where a task is assigned which is tedious and can easily lead to bugs. As an example lets consider a simple function that prints the task’s assignment state:

Finally two points that we should not neglect are readability and scalability. When using UglyTask if we want our code to be readable, in all cases, we have to pass the arguments by their names (thank you Kotlin 🙂 ):

As far as scalability, consider how many changes we need to do to add a new assigned entity. One to the constructor, one to the init function to enforce our business logic and one in every function that we use the task’s state (see: printAssignment()) which adds even more null checks.

Second implementation: The less ugly way

Another way is by having multiple constructors, each for every valid assignment:

This implementation also puts all the logic in the task but it removes those null checks and makes it easier for the developer to understand it:

With that said, all other drawbacks in readability and scalability remain the same:

Final implementation: the sealed classes way 🙂

The best way to implement the business logic is by using Kotlin’s sealed classes. This way we can represent our business logic straight into our code and also keep our code clean, readable and scalable:

Now, printAssignment() leverages all of Kotlin’s powers, including smart cast, making it easier to the eye:

and the rest of the code does not need any extra help like named arguments:

As for scalability, when we want to add a new way of assignment we just extend AssignedTo and we are good to go.