hie-bios only loads lazily components other than main by default

This is a bad default for the debugger (even though it is a good default for HLS?).
It's surprising that

cradle:
  cabal:
     - path: "./"
        component: "lib:hegg"
     - path: "./test/Test.hs"
        component: "test:hegg-test"

doesn't work, but if we instead force hie-bios to load all units eagerly:

cradle:
  cabal:
    component: "all"

it will.

Note that implicit-hie comes up with the former by default. So, by default, a user cannot set a breakpoint in a non-main unit.

Duplicate test which sets breakpoint on internal unit but remove the hie.yaml file in it and make sure it still works.

Since this requires cross cutting changes, it's likely impossible to make for 0.6.0/the MuniHac talk.

It seems we'll have to resort to cradle: cabal: component: "all" for now

It's not so much that the components are loaded lazily, but that only a subset of the components is loaded (depending on the LoadStyle) and any it depends on are pre-compiled and installed in a package-db in hie-bios's cache.

hie-bios expects to be told all other relevant files in the LoadContext, and will give us multiple units at once if we do so.
The problem is that we don't yet know what those files will be, because we ask for the flags at the very beginning. Even trying to delay that call we'd be hard pressed to figure out the file for a function breakpoint without loading the session first.

Shall we add a new LoadStyle to hie-bios so that we can ask to load all components in a multi-cradle at once? Should be possible for multis that are all components in a cabal project, not so obvious otherwise.
(Often this will be the same as loading an "all" component, but could be a strict subset of them, if the hie.yaml doesn't cover them all).

Shall we add a new LoadStyle to hie-bios so that we can ask to load all components in a multi-cradle at once?

Yes, let's do that!

Current `hie-bios` design

Let's briefly think about the current state and then come up with a long term solution.

There are currently two LoadStyles supported by hie-bios

LoadFile: Load the component for the given FilePath
- This is the default, and we should consider this broken as it doesn't satisfy the home unit closure property when multiple components are involved.
LoadWithContext: Load all components for which we have FilePaths
- This ensures the home unit closure property is satisfied but is also "lazy", e.g., reloads whenever new components are "discovered"
- If the cradle can't load multiple files at once, we fall back to LoadFile mode

Both these modes have an additional 'Maybe Component' name, which can be used instead of the file path to indicate what to load exactly.
This mode is used by stack or cradles written like:

cradle:
  cabal:
    component: "lib:hie-bios"

We need this, as some tools can't figure out FilePath -> Component (in particular stack and cabal < 3.2).

This also allows us to trick hie-bios into loading multiple components eagerly via:

cradle:
  cabal:
    component: "all"

Paired with LoadWithContext, this implements an eager component loading of the whole project for cabal only (ignoring benchmarks, tests and executables).

This result in around 5 different ways of setting up the session which feels non-perfect, however, it is important to note that this simply implements the greatest common denominator, allowing us to support stack, cabal and everything else.

New `hie-bios` design

Ok, so how can we improve this?

Form a bird's eye view, I think we want the following ways of loading a hie-bios session:

Lazily load a single file or component with context
- Two major modes:
  1. Legacy mode: ignore context
    - simplest interface, cradle impls should avoid this mode
    - Essentially equivalent to: Maybe Component -> FilePath -> IO ComponentOptions
  2. Use context:
    - Cabal invocation: cabal repl --enable-multi-repl
    - Not supported by: stack, bios, direct (direct is always assumed to be a single module)
    - Can gracefully fall back to Legacy Mode if not supported by cradle
- Lazily means we don't try to load all units in the project but only units that have a filepath witness.
- HLS loads components on demand
- Haskell-debugger cannot load components on demand, so this is virtually broken, except for simple cases.
Eagerly load all specified components
- Equivalent to [Component] -> IO ComponentOptions
- Eagerly means we load all components in a cradle when the first witness is opened.
- Cabal invocation: cabal repl --enable-multi-repl component1 component2 ... componentN
- HLS loads the given components exactly once
- haskell-debugger loads the given components exactly once
- Fallback:
  - Fall back to (1) by calling it with the given filepath witnesses.
Eagerly load project
- Equivalent to IO ComponentOptions
- Eagerly means we load all components in a cradle when the first witness is opened.
- Fallback:
  - Fall back to (1) by calling it with the given filepath witnesses.

This means there are still 4 different ways of loading a project, where only cabal Cradles currently support all session initialisation things.
bios could be easily thaught about it as well, but stack is very tricky.

The final API is:

type Component = String

-- | Always needed for graceful fallback to (1.1) and (1.2)
data TargetWithContext = TargetWithContext 
    { targetFilePath :: FilePath
    , targetContext :: [String]
    }

data CradleLoadMode
    = LoadFile
    | LoadFileTargetWithContext
    | LoadUnitsInferred
    | LoadUnitsFromCradle

getCompilerOptions :: TargetWithContext -> CradleLoadMode -> IO ComponentOptions

We ignore details such as logging, working directory and error handling.

This design mostly affects cabal and perhaps the bios cradle. Stack cradle is unaffected right now.

Cabal examples

Given no hie.yaml, and LoadAllEagerly, this is equivalent to:

cabal repl ... all <filepath> --enable-tests --enable-benchmarks

-- hie.yaml

cradle:
  cabal:
    cabalProject: "./cabal.project.dev"

cabal --project-file ./cabal.project.dev repl ... all <filepath>

-- hie.yaml

cradle:
  cabal:
    cabalProject: "./cabal.project.dev"
    components:
    - path: "./src"
      component: "lib:hie-bios"
    - path: "./exe"
      component: "exe:hie-bios"
    - path: "./tests/BiosTests.hs"
      component: "test:hie-bios"
    - path: "./tests/ParserTests.hs"
      component: "test:parser-tests"

cabal repl ... lib:hie-bios exe:hie-bios test:hie-bios test:parser-tests

-- hie.yaml

cradle:
  cabal:
    component: "lib:hie-bios"

cabal repl ... lib:hie-bios

[Fix #38] Use LoadUnitsFromCradle new hie-bios mode

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hie-bios only loads lazily components other than main by default #38

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alt-romes commented on Sep 1, 2025

Saizan commented on May 7, 2026

fendor commented on May 12, 2026

Current hie-bios design

New hie-bios design

Cabal examples

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