Only run stack if a stack.yaml config is found (#1752)

* Only run stack if a stack.yaml config is found

It is necessary to check for a stack.yaml file to distinguish between
cabal-only projects or stack projects (which are also cabal projects
since stack is built on top of cabal).

* Test that stack is called if stack.yaml exists

ale_linters/haskell/stack_build.vim

@@ -16,7 +16,7 @@ call ale#linter#Define('haskell', {
 \   'name': 'stack_build',
 \   'aliases': ['stack-build'],
 \   'output_stream': 'stderr',
-\   'executable': 'stack',
+\   'executable_callback': 'ale#handlers#haskell#GetStackExecutable',
 \   'command_callback': 'ale_linters#haskell#stack_build#GetCommand',
 \   'lint_file': 1,
 \   'callback': 'ale#handlers#haskell#HandleGHCFormat',

ale_linters/haskell/stack_ghc.vim

@@ -5,7 +5,7 @@ call ale#linter#Define('haskell', {
 \   'name': 'stack_ghc',
 \   'aliases': ['stack-ghc'],
 \   'output_stream': 'stderr',
-\   'executable': 'stack',
+\   'executable_callback': 'ale#handlers#haskell#GetStackExecutable',
 \   'command': 'stack ghc -- -fno-code -v0 %t',
 \   'callback': 'ale#handlers#haskell#HandleGHCFormat',
 \})

autoload/ale/handlers/haskell.vim

@@ -1,5 +1,15 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: Error handling for the format GHC outputs.
+"
+function! ale#handlers#haskell#GetStackExecutable(bufnr) abort
+    if ale#path#FindNearestFile(a:bufnr, 'stack.yaml') isnot# ''
+        return 'stack'
+    endif
+
+    " if there is no stack.yaml file, we don't use stack even if it exists,
+    " so we return '', because executable('') apparently always fails
+    return ''
+endfunction
 
 " Remember the directory used for temporary files for Vim.
 let s:temp_dir = fnamemodify(ale#util#Tempname(), ':h')

test/command_callback/test_haskell_stack_build_command_callback.vader

@@ -0,0 +1,13 @@
+Before:
+  call ale#assert#SetUpLinterTest('haskell', 'stack_build')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The linter should not be executed when there's no stack.yaml file):
+  AssertLinterNotExecuted
+
+Execute(The linter should be executed when there is a stack.yaml file):
+  call ale#test#SetFilename('stack_build_paths/test.hs')
+
+  AssertLinter 'stack', 'stack build --fast'

test/command_callback/test_haskell_stack_ghc_command_callback.vader

@@ -0,0 +1,14 @@
+Before:
+  call ale#assert#SetUpLinterTest('haskell', 'stack_ghc')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The linter should not be executed when there's no stack.yaml file):
+  AssertLinterNotExecuted
+
+Execute(The linter should be executed when there is a stack.yaml file):
+  call ale#test#SetFilename('stack_ghc_paths/test.hs')
+
+  AssertLinter 'stack', 'stack ghc -- -fno-code -v0 %t'
+