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Cats Core · Part 8
I wrote a tiny Todo validator with cats' ValidatedNec and it made one rule click: you can only accumulate errors when the steps don't depend on each other. That's the whole difference between an Applicative and a Monad.
Scala and the JVM · Part 19
A sealed trait removes impossible states from a model, but five details decide whether it holds: constructors, exhaustivity warnings, wildcards, Scala 2 inference and secrets in toString. I compiled each one on Scala 2.13.18.
Learning enumeratum · Part 3
CirceEnum and IntCirceEnum give every enum a JSON codec that follows entryName or value. Decoding is strict by default, and here's how the failures look and how to relax them on purpose.
Learning enumeratum · Part 2
Overriding entryName for legacy strings, naming mixins, enum entries that carry data, and IntEnum/StringEnum, where duplicate codes fail to compile.
Learning enumeratum · Part 1
Three ways to model an enum in Scala 2.13 (Enumeration, sealed traits, and enumeratum), with runnable code, and why my team settled on enumeratum.
Cats Core · Part 7
In Part 4 I wrote map[A, B](fa: Quote[A])(f: A => B) and moved on, but the letters kept slowing me down. A short break from new type classes: two rules that decode most Cats signatures, and the one name that breaks them.
Cats Core · Part 6
What the Nec in ValidatedNec guarantees, why errors.toList failed to compile in my Play controller, and when NonEmptyList is the better container.
Cats Core · Part 5
A price and a delivery cost are both optional, and nested map gives me Some(Some(15)). Apply combines independent values into one result, and Applicative adds the starting value a whole cart needs.
Scala and the JVM · Part 18
I replaced this site's Spring Boot backend with Scala 2.13, Play 3, Anorm and Twirl in one pull request. The rewrite was the easy part: this is how I fixed the contracts, kept the outbox wire format, and rehearsed the rollback on a restored production backup.
Cats Core · Part 4
List and Option already know how to map. I give my own Quote a Functor instance, reuse the same discount function, and trace why identity and composition matter.