Implement code formatting #42

#38 is a discussion of possible approaches. This issue tracks implementation progress for the current approach.

• add CLI subcommand with options and file processing Add format CLI subcommand #46
  • format directories implied by gren.json Add format CLI subcommand #46
  • --yes Add format CLI subcommand #46
  • --validate Implement gren format --validate #143
  • explicit INPUTS... Add format CLI subcommand #46
  • --stdin Add format CLI subcommand #46
• modify parser/AST to retain comments and necessary whitespace info
  • modify whitespace parser to parse comments Whitespace parser parses comments #56
  • Formatting retains comments (module header and paren expressions) #132
  • format: retain comments in import statements #142
  • format: retain top-level comments #153
  • format: retain comments in expression bodies (value decls, let, lambda) #154
  • format: retain comments in case and if expressions #159
  • format: retain comments in arrays and records #160
  • format: retain comments in remaining expressions #161
  • format: retain comments in remaining types #162
  • format: good things to fix before 0.2 #165
  • TBD: retain comments part N
  • retain comments in empty arrays and records
• implement the formatting logic
  • initial implementation (everything no requiring whitespace info should work) Implement formatting logic (without comments/linebreak support) #117
  • allow kernel module features when formatting kernel modules
  • TBD any adjustments to formatting rules
    • comments at Format source code with current gren format example-projects#11
  • TBD fix anything that's broken
    • format: (kernel modules) keep infix operators in the original order #164
• add formatting test suite
  • basic pattern established in Formatting retains comments (module header and paren expressions) #132
  • (maybe) a property test to ensure that formatting is idempotent
• cleanup
  • extract Text.PrettyPrint.Avh4 to a separate library to share with elm-format

Planned CLI options:

gren format [INPUTS... | --stdin] [--yes] [--validate]

• INPUTS: zero or more files to format, or directories to recursively search for gren files to format. If none are given, use the directories implied by gren.json
• --stdin: instead of formatting files, read stdin and format it to stdout
• --yes: don't prompt for confirmation before formatting files and overwriting them (does nothing with --stdin)
• --validate: Don't modify files, and exit 0 if files are all formatted, or exit 1 if some files are not formatted. Intended for use on CI.