Add a function for computing the number of arguments for a function

autoload/ale/util.vim

@@ -123,3 +123,18 @@ function! ale#util#GetMatches(lines, patterns) abort
 
     return l:matches
 endfunction
+
+" Given the name of a function, a Funcref, or a lambda, return the number
+" of named arguments for a function.
+function! ale#util#FunctionArgCount(function) abort
+    let l:Function = ale#util#GetFunction(a:function)
+
+    redir => l:output
+        silent function Function
+    redir END
+
+    let l:match = matchstr(split(l:output, "\n")[0], '\v\([^)]+\)')[1:-2]
+    let l:arg_list = filter(split(l:match, ', '), 'v:val !=# ''...''')
+
+    return len(l:arg_list)
+endfunction

test/test_function_arg_count.vader

@@ -0,0 +1,41 @@
+Before:
+  function! Func0()
+  endfunction
+  function! Func1(x)
+  endfunction
+  function! Func2(x,y)
+  endfunction
+  function! Func3(x,y,z)
+  endfunction
+  function! Func3a(x,y,z,...)
+  endfunction
+
+After:
+  delfunction Func0
+  delfunction Func1
+  delfunction Func2
+  delfunction Func3
+  delfunction Func3a
+
+Execute(We should be able to compute the argument count for function names):
+  AssertEqual 0, ale#util#FunctionArgCount('Func0')
+  AssertEqual 1, ale#util#FunctionArgCount('Func1')
+  AssertEqual 2, ale#util#FunctionArgCount('Func2')
+  AssertEqual 3, ale#util#FunctionArgCount('Func3')
+  AssertEqual 3, ale#util#FunctionArgCount('Func3a')
+
+Execute(We should be able to compute the argument count for Funcrefs):
+  AssertEqual 0, ale#util#FunctionArgCount(function('Func0'))
+  AssertEqual 1, ale#util#FunctionArgCount(function('Func1'))
+  AssertEqual 2, ale#util#FunctionArgCount(function('Func2'))
+  AssertEqual 3, ale#util#FunctionArgCount(function('Func3'))
+  AssertEqual 3, ale#util#FunctionArgCount(function('Func3a'))
+
+Execute(We should be able to compute the argument count for lambdas):
+  if has('lambda')
+    AssertEqual 0, ale#util#FunctionArgCount({->1})
+    AssertEqual 1, ale#util#FunctionArgCount({x->1})
+    AssertEqual 2, ale#util#FunctionArgCount({x,y->1})
+    AssertEqual 3, ale#util#FunctionArgCount({x,y,z->1})
+    AssertEqual 3, ale#util#FunctionArgCount({x,y,z,...->1})
+  endif