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Haskell Starter-Kit

This project is a dynamic template for a modern Haskell development environment. With a single command it bootstraps a haskell.nix project, shell with rich ecosystem (hlint, ghcid), out of the box support for profiling, Emacs integration and many more.

Key features

  • Latests building environment
    • GHC (8.10.2)
    • Cabal (3.2.0.0)
    • ghcid
  • Linter support
    • hlint (3.2.1)
    • apply-refact
    • pointfree
    • weeder (2.1.3)
  • Formatters
    • stylish-haskell (0.12.2.0)
    • cabal-fmt
  • Testing environment for both unit and property-based tests
    • hspec
    • hedgehog
  • Other
    • local Hoogle server
    • direnv
    • sleek Emacs integration

How to create new project using this template

pre-requirement: you have Nix installed on your system

nix-shell -p cookiecutter git --run 'cookiecutter gh:EncodePanda/haskell-starter-kit

Set defaults

Reentering the defaults (like github user name) might be tedious and boring. That's why it is a good practice to setup the defaults. Modify ~/.cookiecutterrc by providing default value for github user and author name. In my case this is:

default_context:
  "gh_user": "EncodePanda"
  "author_name": "Paweł Szulc"

Example

1. Create the project using template

➜  ~ nix-shell -p cookiecutter git --run 'cookiecutter gh:EncodePanda/haskell-template-project'
You've downloaded /Users/rabbit/.cookiecutters/haskell-template-project before. Is it okay to delete and re-download it? [yes]:
project_name [Enter project name]: foo
project_synopsis [Enter project synopsis]: Example project
module [Foo]:
category [Unclassified]: Example
author_name [Paweł Szulc]:
gh_user [EncodePanda]:
formatter_indent [2]:
formatter_columns [110]: 80

2. Enter nix-shell

note: Before entering the nix-shell consider reading README.md, there is a section about using Nix cache - so that you don't have to recompile the whole world

➜  ~ cd foo
➜  foo nix-shell
trace: gitSource.nix: /Users/rabbit/foo does not seem to be a git repository,
assuming it is a clean checkout.
trace: Using latest index state for foo!
(...)
trace: Using latest index state for cabal-install!
trace: Using index-state: 2020-09-14T00:00:00Z for cabal-install

3. Build the project

[nix-shell:~/foo]$ cabal build
Resolving dependencies...
Build profile: -w ghc-8.10.2 -O1
(...)
Building library for foo-0.1.0.0..
[1 of 1] Compiling Lib              ( src/Lib.hs, /Users/rabbit/foo/dist-newstyle/build/x86_64-osx/ghc-8.10.2/foo-0.1.0.0/build/Lib.o, /Users/rabbit/foo/dist-newstyle/build/x86_64-osx/ghc-8.10.2/foo-0.1.0.0/build/Lib.dyn_o )
(...)
[1 of 1] Compiling Main             ( app/Main.hs, /Users/rabbit/foo/dist-newstyle/build/x86_64-osx/ghc-8.10.2/foo-0.1.0.0/x/foo-exe/build/foo-exe/foo-exe-tmp/Main.o )
Linking /Users/rabbit/foo/dist-newstyle/build/x86_64-osx/ghc-8.10.2/foo-0.1.0.0/x/foo-exe/build/foo-exe/foo-exe ...

4. Run tests

[nix-shell:~/foo]$ cabal test --test-show-details=streaming
Build profile: -w ghc-8.10.2 -O1
(...)
Running 1 test suites...
Test suite foo-test: RUNNING...

Foo.Example
  unit tests
    should work
  hedgehog property tests
    should work

Finished in 0.0022 seconds
2 examples, 0 failures
Test suite foo-test: PASS
Test suite logged to:
1 of 1 test suites (1 of 1 test cases) passed.

5. Use linter

look for issues, non found

[nix-shell:~/foo]$ hlint-all
No hints

introduce code that is not perfect

[nix-shell:~/foo]$ echo "foo :: String -> Int" >> src/Lib.hs; echo "foo = \_ -> 10" >> src/Lib.hs

[nix-shell:~/foo]$ cat src/Lib.hs
module Lib where
foo :: String -> Int
foo = \_ -> 10

look for issues one more time, found it!

[nix-shell:~/foo]$ hlint-all
src/Lib.hs:3:1-14: Warning: Redundant lambda
Found:
  foo = \ _ -> 10
Perhaps:
  foo _ = 10

src/Lib.hs:3:7-14: Suggestion: Use const
Found:
  \ _ -> 10
Perhaps:
  const 10

2 hints

ask linter to automatically fix them

[nix-shell:~/foo]$ hlint-all-fix

[nix-shell:~/foo]$ cat src/Lib.hs
module Lib where
foo :: String -> Int
foo _ = 10

[nix-shell:~/foo]$ hlint-all
No hints

6. Use ghcid

[nix-shell:~/projects/Foo]$ ghcid
Loading cabal repl ...
Build profile: -w ghc-8.10.1 -O1
In order, the following will be built (use -v for more details):
 - Foo-0.1.0.0 (lib) (ephemeral targets)
Preprocessing library for Foo-0.1.0.0..
GHCi, version 8.10.1: https://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Lib              ( src/Lib.hs, interpreted )
Ok, one module loaded.

All good (1 module, at 23:56:46)

7. Hoogle support

[nix-shell:~/projects/foo]$ hoogle server --local
Server started on port 8080
Reading log...0.00s
2021-01-05T15:39:39.488174 - Server starting on port 8080 and host/IP Host "127.0.0.1"

8. Pointfree support

You can use pointfree in the shell

[nix-shell:~/projects/foo]$ pointfree "foo list = fmap (+1) list"
foo = fmap (1 +)

or you can use it in the REPL

[nix-shell:~/projects/foo]$ cabal repl
Loaded GHCi configuration from /Users/rabbit/projects/foo/.ghci
[1 of 1] Compiling Lib              ( src/Lib.hs, interpreted )
Ok, one module loaded.

λ> :pf foo list = fmap (+1) list
foo = fmap (1 +)
*Lib

9. Weeder support

[nix-shell:~/projects/foo]$ weeder
Weeds detected: 0

[nix-shell:~/projects/foo]$ echo "notUsed :: Int" >> src/Lib.hs
[nix-shell:~/projects/foo]$ echo "notUsed = 10" >> src/Lib.hs

[nix-shell:~/projects/foo]$ cat src/Lib.hs
module Lib where
notUsed :: Int
notUsed = 10

[nix-shell:~/projects/foo]$ cabal build
(...)
Preprocessing executable 'foo-exe' for foo-0.1.0.0..
Building executable 'foo-exe' for foo-0.1.0.0..
(...)
Linking ~/projects/foo/dist-newstyle/build/x86_64-osx/ghc-8.10.2/

[nix-shell:~/projects/foo]$ weeder
src/Lib.hs:3:1: error: notUsed is unused

       1 ┃ module Lib where
       2 ┃ notUsed :: Int
       3 ┃ notUsed = 10

    Delete this definition or add ‘Lib.notUsed’ as a root to fix this error.

Weeds detected: 1