nix-playgraound/notes/048-haskell-time-windows.md

Haskell Time Windows

This note covers 45-haskell-time-windows/, which uses Data.Time to parse UTC timestamps and classify maintenance windows.

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1. Why This Exists Next to the QuickCheck Window Example

26-haskell-quickcheck/ works with abstract intervals and normalization rules.

This example teaches a different concern:

• parsing real UTC timestamps,
• comparing them against concrete windows, and
• rendering the resulting status in terms people would actually read.

The shared domain is intentional, but the concept is different.

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2. What the Status Function Returns

statusAt sorts the windows by start time, then returns one of three cases:

• Active, with the remaining time in the current window,
• Waiting, with the delay until the next window starts, or
• NoMoreWindows.

That is the useful pattern to remember. Time logic often becomes easier to explain once the raw comparison result is turned into a small sum type.

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3. Why the Test Checks Exact Delays

The test does not stop at string formatting. It also checks exact NominalDiffTime values:

• 1800 seconds for an active window with 30 minutes left, and
• 2700 seconds for a future window starting in 45 minutes.

That matters because time code can render the right words while still comparing the wrong timestamps underneath.

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4. Commands to Try

cd 45-haskell-time-windows

nix develop
cabal run
cabal run -- 2026-05-05T11:45:00Z
cabal test

nix build
./result/bin/mini-time-windows 2026-05-05T11:45:00Z

nix run . -- 2026-05-05T11:45:00Z
nix flake check