Haskell Debugger

We are working on a first class debugger for Haskell!

Installation

Please find up to date installation instructions on the project homepage!

Warning

hdb can currently be compiled with the 9.14 alpha pre-releases or with a nightly version The first release it will be compatible with is GHC 9.14.

To install and use the debugger, you need the executable hdb and the VSCode extension Haskell Debugger.

Since hdb implements the Debug Adapter Protocol (DAP), it also supports debugging with tools such as vim, neovim, or emacs -- as long as a DAP client is installed and the launch arguments for hdb configured.

To run the debugger, the same version of GHC which compiled it needs to be in PATH. Make sure the DAP client knows this. For instance, to launch VSCode with a specific GHC use:

PATH=/path/to/ghc-dir:$PATH code /path/to/proj

Usage

To use the debugger in VSCode, select the debugger tab, select Haskell Debugger, and create a launch.json file by clicking the debugger settings icon (next to the green run button). Now, it is also supported to just Run a file which contains a main function.

The launch.json file contains some settings about the debugger session here. Namely:

Setting	Description
projectRoot	the full path to the project root. this is typically ${workspaceFolder}, a value which is interpolated by the editor with the actual path
entryFile	the relative path from the project root to the file with the entry point for execution
entryPoint	the name of the function that is called to start execution
entryArgs	the arguments passed to the entryPoint. If the entryPoint is main, these arguments are passed as environment arguments (as in getArgs) rather than direct function arguments.
extraGhcArgs	additional flags to pass to the ghc invocation that loads the program for debugging.

Change them accordingly.

To run the debugger, simply hit the green run button. See the Features section below for what is currently supported.

Multiple home units session

Multiple home units is supported but currently may require a workaround (issue is tracked by #38).

If your multiple home units session does not work by default (e.g. if you cannot set breakpoints on different units), and you do not have a hie.yaml file, you may want to try creating a hie.yaml file in the root of the workspace with:

cradle:
    cabal:
        component: "all"

Related Work

hdb is inspired by the original haskell-debug-adapter by @phoityne.

hdb improves on the original ideas implemented in haskell-debug-adapter but makes them more robust by implementing the debugger directly via the GHC API (similarly to HLS), rather than by communicating with a custom ghci process.

We have been doing custom work on GHC to support debugging in a predictable, robust, and more performant way. That is why hdb is only compatible with the latest and greatest GHC. If you want to debug using an older GHC version (9.12 and older), please check out haskell-debug-adapter.

To implement the Debug Adapter Protocol (DAP) server part, we are using the dap library by @dmjio. dap is a framework for building language-agnostic DAP.

The hdb is transparently compatible with most projects (simple, Cabal, Stack, custom hie.yaml) because it uses hie-bios to figure out the right flags to prepare the GHC session with.

Features

Many not listed! Here are a few things:

Stepping

• Continue (resume execution forward)
• Next (step within local function)
• Step into (single step to next immediate tick)
• Step out (execute until end of function and break after the call)

In Reverse

• Local step backwards (ie reverse of Next)
• Single step backwards (ie reverse of Step into)
• Continue backwards (resume execution backwards until a breakpoint is hit)

Breakpoints

• Module breakpoints
• Function breakpoints
• Exception breakpoints
• Data breakpoints
• Instruction breakpoints

Conditionals

• Conditional breakpoints (breakpoint is hit only if condition is satisfied)
• Hit conditional breakpoints (stop after N number of hits)

Building from source

To build hdb:

cabal build -w /path/to/recent/ghc exe:hdb

To build the VSCode extension

cd vscode-extension
nix-build

Testing

cd test/integration-tests
make GHC=/path/to/recent/ghc \
     DEBUGGER=$(cd ../.. && cabal list-bin -w /path/to/recent/ghc exe:hdb)