What the imperative shell of an Functional Core/Imperative Shell language looks like

• Pipefish

  36 138 9.4 Go

  Source code for the Pipefish programming language

No, it's "shell" as in "shell of the code". The idea is that the imperative bits of the language, the bits that do the mutation of state and the IO, can can call lovely pure referentially transparent functions. But functions can't call commands (otherwise by definition they wouldn't be pure). So all your imperative-ness is reduced to about 1% of your code which lives right at the top of your call stack --- the "imperative shell" of your code. See [here](https://github.com/tim-hardcastle/Charm/blob/main/examples/adv.ch) for an example. The "imperative shell" is the main function --- all 13 lines of it --- and everything everywhere else is pure and immutable.

• flix

  11 2,054 9.9 Scala

  The Flix Programming Language

  

I like it. Modern languages that distinguish between pure and impure programs like Flix, Koka, and Effekt do so on the type level instead of syntactically. This has three advantages:

• WorkOS

  workos.com sponsored

  The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.

  

• effekt

  13 288 9.7 Scala

  A research language with effect handlers and lightweight effect polymorphism

  

I like it. Modern languages that distinguish between pure and impure programs like Flix, Koka, and Effekt do so on the type level instead of syntactically. This has three advantages:

• promises-spec

  22 1,830 0.0

  An open standard for sound, interoperable JavaScript promises—by implementers, for implementers.

  

Advantage 1, nesting, is the most important here, and it's often the most-overlooked advantage. Overlooking nesting is how Promises in Javascript got to be fundamentally broken.

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