Add linters for Elixir (#113)

* Add Credo linter for Elixir

* Add requested changes

TODO: check if all message types are covered in `if` chain.

* Add information about Credo linter to README

* Add information about Credo linter to doc

3 files changed, 54 insertions, 1 deletions

diff --git a/README.md b/README.md
index 1e414095..f159e075 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,7 @@ name. That seems to be the fairest way to arrange this table.
 | CSS | [csslint](http://csslint.net/) |
 | Cython (pyrex filetype) | [cython](http://cython.org/) |
 | D | [dmd](https://dlang.org/dmd-linux.html)^ |
+| Elixir | [credo](https://github.com/rrrene/credo) |
 | Fortran | [gcc](https://gcc.gnu.org/) |
 | Go | [gofmt -e](https://golang.org/cmd/gofmt/), [go vet](https://golang.org/cmd/vet/), [golint](https://godoc.org/github.com/golang/lint) |
 | Haskell | [ghc](https://www.haskell.org/ghc/) |
@@ -47,7 +48,7 @@ name. That seems to be the fairest way to arrange this table.
 | PHP | [php -l](https://secure.php.net/), [phpcs](https://github.com/squizlabs/PHP_CodeSniffer) |
 | Pug | [pug-lint](https://github.com/pugjs/pug-lint) |
 | Python | [flake8](http://flake8.pycqa.org/en/latest/) |
-| Ruby   | [rubocop](https://github.com/bbatsov/rubocop) |
+| Ruby | [rubocop](https://github.com/bbatsov/rubocop) |
 | SASS | [sass-lint](https://www.npmjs.com/package/sass-lint) |
 | SCSS | [sass-lint](https://www.npmjs.com/package/sass-lint), [scss-lint](https://github.com/brigade/scss-lint) |
 | Scala | [scalac](http://scala-lang.org) |
diff --git a/ale_linters/elixir/credo.vim b/ale_linters/elixir/credo.vim
new file mode 100644
index 00000000..0d84fcd9
--- /dev/null
+++ b/ale_linters/elixir/credo.vim
@@ -0,0 +1,51 @@
+" Author: hauleth - https://github.com/haulethe
+
+if exists('g:loaded_ale_linters_elixir_credo')
+  finish
+endif
+
+let g:loaded_ale_linters_elixir_credo = 1
+
+function! ale_linters#elixir#credo#Handle(buffer, lines)
+  " Matches patterns line the following:
+  "
+  " stdin:19: F: Pipe chain should start with a raw value.
+  let l:pattern = '\v^stdin:(\d+):?(\d+)?: (.): (.+)$'
+  let l:output = []
+
+  for l:line in a:lines
+    let l:match = matchlist(l:line, l:pattern)
+
+    if len(l:match) == 0
+        continue
+    endif
+
+    let l:type = l:match[3]
+    let l:text = l:match[4]
+
+    if l:type ==# 'C'
+      let l:type = 'E'
+    elseif l:type ==# 'R'
+      let l:type = 'W'
+    endif
+
+    " vcol is Needed to indicate that the column is a character.
+    call add(l:output, {
+    \   'bufnr': a:buffer,
+    \   'lnum': l:match[1] + 0,
+    \   'vcol': 0,
+    \   'col': l:match[2] + 0,
+    \   'type': l:type,
+    \   'text': l:text,
+    \   'nr': -1,
+    \})
+  endfor
+
+  return l:output
+endfunction
+
+call ale#linter#Define('elixir', {
+      \ 'name': 'credo',
+      \ 'executable': 'mix',
+      \ 'command': 'mix credo suggest --format=flycheck --read-from-stdin',
+      \ 'callback': 'ale_linters#elixir#credo#Handle' })
diff --git a/doc/ale.txt b/doc/ale.txt
index 6f47cc2b..07cfec31 100644
--- a/doc/ale.txt
+++ b/doc/ale.txt
@@ -56,6 +56,7 @@ The following languages and tools are supported.
 * CSS: 'csslint'
 * Cython (pyrex filetype): 'cython'
 * D: 'dmd'
+* Elixir: 'credo'
 * Fortran: 'gcc'
 * Go: 'gofmt -e', 'go vet', 'golint'
 * Haskell: 'ghc'