Debug Haskell Bazel Targets

This skill helps diagnose and fix common Haskell Bazel build errors, particularly dependency visibility and configuration issues.

Common Error Pattern: Dependency Visibility

Symptom

ERROR: in _haskell_test rule //path/to:target: target '@@rules_haskell~~stack_snapshot~stackage//:base' is not visible from target '//path/to:target'

Root Cause

Core Haskell packages (base, bytestring, containers, directory, filepath, text, time, etc.) should NOT be imported from @stackage//. They are part of the GHC distribution and must be referenced directly.

Solution Pattern

WRONG:

haskell_test(
    name = "unit-tests",
    srcs = glob(["test/**/*.hs"]),
    deps = [
        ":rpg-ruleset-core",
        "@stackage//:QuickCheck",
        "@stackage//:base",        # L WRONG
        "@stackage//:hspec",
        "@stackage//:text",        # L WRONG
    ],
)

CORRECT:

haskell_test(
    name = "unit-tests",
    srcs = glob(["test/**/*.hs"]),
    deps = [
        ":rpg-ruleset-core",
        "//:base",                 #  CORRECT
        "//:text",                 #  CORRECT
        "@stackage//:QuickCheck",  #  Third-party packages use @stackage//
        "@stackage//:hspec",
    ],
)

Core vs Third-Party Dependencies

Core GHC Packages (use //:package-name)

• //:base
• //:bytestring
• //:containers
• //:directory
• //:filepath
• //:text
• //:time
• //:array
• //:deepseq
• //:process
• //:mtl
• //:transformers

Third-Party Packages (use @stackage//:package-name)

• @stackage//:aeson
• @stackage//:yaml
• @stackage//:cmark
• @stackage//:QuickCheck
• @stackage//:hspec
• @stackage//:http-conduit
• @stackage//:optparse-applicative
• Any package not bundled with GHC

Debugging Workflow

When encountering build errors:

1. Identify the error type

   • Visibility errors � Check dependency prefixes (//: vs @stackage//)
   • Missing dependencies � Check if package is in MODULE.bazel
   • Compilation errors � Check Haskell syntax and imports

2. For visibility errors:

   # Check which dependencies are causing issues
   grep -r "@stackage//:base" haskell/
   grep -r "@stackage//:text" haskell/
   grep -r "@stackage//:containers" haskell/
   

3. Fix all BUILD.bazel files:

   • Replace @stackage//:base � //:base
   • Replace @stackage//:text � //:text
   • Replace @stackage//:containers � //:containers
   • Keep @stackage// for third-party packages

4. Verify consistency across library and test targets:

   • haskell_library deps should match haskell_test deps for core packages
   • Both should use //: prefix for GHC-bundled packages

5. Rebuild:

   bazel clean
   bazel build //...
   bazel test //...
   

Quick Reference

# Template for Haskell library/binary/test
haskell_library(
    name = "my-lib",
    srcs = glob(["src/**/*.hs"]),
    src_strip_prefix = "src",
    visibility = ["//visibility:public"],
    deps = [
        # Core GHC packages (no @stackage)
        "//:base",
        "//:bytestring",
        "//:containers",
        "//:directory",
        "//:filepath",
        "//:text",
        "//:time",

        # Third-party packages (with @stackage)
        "@stackage//:aeson",
        "@stackage//:yaml",
        "@stackage//:cmark",
    ],
)

haskell_test(
    name = "unit-tests",
    srcs = glob(["test/**/*.hs"]),
    src_strip_prefix = "test",
    deps = [
        ":my-lib",

        # Core GHC packages
        "//:base",
        "//:text",

        # Test frameworks (third-party)
        "@stackage//:QuickCheck",
        "@stackage//:hspec",
    ],
)

Common Error: Missing Test Files

Symptom

ERROR: No source file defining the main module 'Main'.
You may need to set the 'main_file' attribute or the 'module_name' attribute in a 'haskell_module' rule.

Root Cause

The haskell_test rule expects actual Haskell test files (*.hs) but the test directory may only contain fixtures or be empty.

Solution

1. Remove the test target if no tests exist yet:

   # Remove or comment out until tests are written
   # haskell_test(
   #     name = "unit-tests",
   #     srcs = glob(["test/**/*.hs"]),
   #     ...
   # )
   

2. Or create a minimal test file (test/Main.hs):

   module Main where
   main :: IO ()
   main = putStrLn "Tests not implemented yet"
   

Common Error: Custom Dependency Glob Pattern Mismatch

Symptom

Build fails or can't find source files when using custom dependencies from non_module_deps.bzl

Root Cause

The strip_prefix in http_archive changes the directory structure, but the glob pattern in build_file_content doesn't match the post-strip structure.

Solution Pattern

When using strip_prefix:

http_archive(
    name = "libyaml",
    urls = ["https://hackage.haskell.org/package/libyaml-0.1.4/libyaml-0.1.4.tar.gz"],
    strip_prefix = "libyaml-0.1.4",  # ← Directory is stripped
    build_file_content = """
haskell_cabal_library(
    name = "libyaml",
    srcs = glob(["**/*"]),  # ✓ Use glob without directory prefix
    ...
)
"""
)

Without strip_prefix:

http_archive(
    name = "yaml",
    urls = ["https://hackage.haskell.org/package/yaml-0.11.11.2/yaml-0.11.11.2.tar.gz"],
    # No strip_prefix ← Directory preserved
    build_file_content = """
haskell_cabal_library(
    name = "yaml",
    srcs = glob(["yaml-0.11.11.2/**/*"]),  # ✓ Include directory in glob
    ...
)
"""
)

Debugging tip: Check the actual directory structure in the external repository:

ls -la ~/.bazel/external/_main~non_module_deps~<package-name>/

Additional Notes

• The error message mentions @@rules_haskell~~stack_snapshot~stackage - this is Bazel's canonical repository name
• GHCi warnings about missing binary are informational and don't affect builds
• Use --verbose_failures flag for detailed error information
• After fixing deps, run bazel clean if caching causes issues
• For custom dependencies, verify glob patterns match the directory structure after strip_prefix is applied

When to Use This Skill

Invoke this skill when:

• Encountering "not visible from target" errors with Haskell dependencies
• Setting up new haskell_library, haskell_binary, or haskell_test targets
• Migrating existing Haskell code to Bazel
• Debugging BUILD.bazel dependency configuration