nix-playgraound/notes/026-haskell-maybe-and-either.md

Haskell Maybe and Either

This note covers 23-haskell-maybe-either/, which builds a release request from key=value inputs by using Maybe for optional data and Either for validation failures.

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1. Why Both Types Matter

Maybe answers one question: is a value present or absent?

Either answers a different question: if something failed, why?

This example uses both on purpose:

• optional fields such as owner stay as Maybe,
• required fields start as lookups that may be absent, and
• missing or invalid required data becomes Either String ... with an error message.

That gives the program a clean progression from raw input toward a validated domain value.

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2. Where Maybe Shows Up

The input is a flat list of assignments such as service=api and owner=platform.

lookupOptional searches that list and returns Maybe String:

lookupOptional :: String -> [(String, String)] -> Maybe String

That is the right level for values that are genuinely optional. In the example, owner can be absent without making the whole request invalid.

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3. Where Either Takes Over

Required fields cannot stay as Maybe, because the rest of the program needs a reason when construction fails.

The example upgrades a missing field into an error:

lookupRequired :: String -> [(String, String)] -> Either String String

It then adds more validation:

• environment parsing,
• replica count parsing, and
• canary percentage rules for the rollout strategy.

That keeps absence and validation separate, which makes the control flow easier to read.

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4. Commands to Try

cd 23-haskell-maybe-either

nix develop
cabal run
cabal run -- service=api env=production replicas=3 strategy=canary canary=10 owner=platform
cabal test

nix build
./result/bin/mini-release-request service=api env=production replicas=3 strategy=canary canary=10 owner=platform

nix run . -- service=api env=production replicas=3 strategy=canary canary=10 owner=platform
nix flake check