Convert validate.sh to a Haskell script

Makefile

@@ -29,16 +29,16 @@ init: ## Set up git hooks and ignored revisions
 
 .PHONY: style
 style: ## Run the code styler
-	@fourmolu -q -i Cabal Cabal-syntax cabal-install
+	@fourmolu -q -i Cabal Cabal-syntax cabal-install cabal-validate
 
 .PHONY: style-modified
 style-modified: ## Run the code styler on modified files
-	@git ls-files --modified Cabal Cabal-syntax cabal-install \
+	@git ls-files --modified Cabal Cabal-syntax cabal-install cabal-validate \
 		| grep '.hs$$' | xargs -P $(PROCS) -I {} fourmolu -q -i {}
 
 .PHONY: style-commit
 style-commit: ## Run the code styler on the previous commit
-	@git diff --name-only HEAD $(COMMIT) Cabal Cabal-syntax cabal-install \
+	@git diff --name-only HEAD $(COMMIT) Cabal Cabal-syntax cabal-install cabal-validate \
 		| grep '.hs$$' | xargs -P $(PROCS) -I {} fourmolu -q -i {}
 
 # source generation: SPDX

cabal-validate/README.md

@@ -0,0 +1,23 @@
+# cabal-validate
+
+`cabal-validate` is a script that builds and tests `Cabal` and `cabal-install`.
+`cabal-validate` can be run with `validate.sh` in the repository root;
+arguments passed to `validate.sh` will be forwarded to `cabal-validate`.
+
+Notable arguments include:
+
+- `-v`/`--verbose` to display build and test output in real-time, instead of
+  only if commands fail.
+- `-s`/`--step` to run a specific step (e.g. `-s build -s lib-tests` will only
+  run the `build` and `lib-tests` steps).
+- `-p`/`--pattern` to filter tests by a pattern.
+
+## Hacking on cabal-validate
+
+Overview of important modules:
+
+- `Main.hs` encodes all the commands that are run for each step.
+- `Cli.hs` parses the CLI arguments and resolves default values from the
+  environment, like determining which steps are run by default or the `--jobs`
+  argument to pass to test suites.
+- `Step.hs` lists the available steps.

cabal-validate/cabal-validate.cabal

@@ -0,0 +1,47 @@
+cabal-version: 3.0
+name:          cabal-validate
+version:       1.0.0
+copyright:     2024-2024, Cabal Development Team (see AUTHORS file)
+license:       BSD-3-Clause
+author:        Cabal Development Team <[email protected]>
+synopsis:      An internal tool for building and testing the Cabal package manager
+build-type:    Simple
+
+common common
+    ghc-options: -Wall
+
+    if impl(ghc <9.6)
+        -- Pattern exhaustiveness checker is not as good, misses a case.
+        ghc-options: -Wno-incomplete-patterns
+
+    default-language: Haskell2010
+    default-extensions:
+        OverloadedStrings
+        , TypeApplications
+
+executable cabal-validate
+    import: common
+    ghc-options: -O -threaded -rtsopts -with-rtsopts=-N
+
+    main-is: Main.hs
+    hs-source-dirs: src
+
+    other-modules:
+        , Cli
+        , ClockUtil
+        , OutputUtil
+        , ProcessUtil
+        , Step
+
+    build-depends:
+        , base >=4 && <5
+        , ansi-terminal >=1 && <2
+        , bytestring >=0.11 && <1
+        , containers >=0.6 && <1
+        , directory >=1.0 && <2
+        , filepath >=1 && <2
+        , optparse-applicative >=0.18 && <1
+        , terminal-size >=0.3 && <1
+        , text >=2 && <3
+        , time >=1 && <2
+        , typed-process >=0.2 && <1