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Agent-first Haskell DSL for TypeSafe's Jev judgment model: typed packets, inferred types, answers under the same labels
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CommunityJev Repos & SDKs
Jev Dsl
Early-alpha Haskell DSL for building Jev question packets as expressions with inferred types, exact request rendering, and exhaustive typed answer handlers.
- Stars
- 8
- Language
- Haskell
- License
- MIT
- Published by
- Community
- Author
- inanna-malick
- Last verified
- Oct 2026
About this project
A Haskell DSL for agents that would rather have a question judged than guessed. A packet of labelled questions is written once as an expression; its type is inferred, it renders to the exact request JSON for TypeSafe's Jev, and the answers come back under the same labels as records read by field. A choice is consumed through exhaustive labelled handlers or its carried payload, with a policy when needed. The branch that runs is one the program wrote and carries the payload it was offered with. No schema, no…
Quoted from the project's README. Read the full README.
Highlights
- Single Jev.Operators surface with no schema, instance, codec, or network code; every example is compiled.
- Packet types are inferred from questions, with nesting flattened to dotted wire keys and read back by label.
- Choice answers settle only through exhaustive labelled handlers carrying program-chosen payloads, never parsed strings.
- Compile errors are a tested interface phrasing mistakes in author vocabulary, with named lenient, careful, and strict policies.
Watch out
Early alpha interface that is still moving and has only been used in its own repository; check the status section before adopting. Copy reviewed against the repository README on 2026-10-04, not from running it.
Questions about this entry
Short answers drawn from the linked project or page.
The packet type is inferred from the expression, printable for inspection, and enforced at compile time: duplicate labels, missing handlers, and wrong cells fail to compile with author-worded errors instead of failing at runtime.
It bounds the damage rather than removing it: text in the state can still steer which alternative wins, but the answer can only move among branches the program wrote and payloads it offered, never a new branch.
Use the three named policies over mass, margin, and confidence: lenient for read-only choices, careful for starting work, strict for anything with a receipt; unmet policies return a structured reason instead of a verdict.
A language-model author: one import, no classes or schemas to memorize, handlers found by label in any order, shared question sets as ordinary values, and a model-focused authoring guide with compiled examples.
Three and nothing else. Noul asks about a proposition and returns one probability that it holds. Choice asks which labelled alternative applies and returns the winner, a mass for every alternative, and a confidence. Score asks where something falls on an ordered rubric of one to ten levels and returns a mass per level plus an expectation. There is no prompt and no generation step anywhere in the loop.
Jev.Operators is the entire surface: no schema, no instance declarations, no codec and no network code. It is one import. Jev.Core is the same library made polymorphic over the JSON type for an environment that cannot load aeson, and every example in the README is compiled, either by test/Readme.hs or as part of examples/Guard.hs, so the documentation cannot drift from the code.
There is no format to coax and no retry loop for malformed output, because the answer to a choice is one of the keys you sent. The README is careful that this does not make the state trustworthy: text in the state can still steer which alternative wins. What it does is bound the damage, since a hostile state can move the answer among branches the program wrote but never introduce one it did not.
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