patch 9.0.1773: cannot distinguish Forth and Fortran *.f files

Problem:  cannot distinguish Forth and Fortran *.f files
Solution: Add Filetype detection Code

Also add *.4th as a Forth filetype

closes: #12251

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Doug Kearns <dougkearns@gmail.com>

runtime/autoload/dist/ft.vim

@@ -287,6 +287,37 @@ export def FTe()
   endif
 enddef
 
+def IsForth(): bool
+  var first_line = nextnonblank(1)
+
+  # SwiftForth block comment (line is usually filled with '-' or '=') or
+  # OPTIONAL (sometimes precedes the header comment)
+  if getline(first_line) =~? '^\%({\%(\s\|$\)\|OPTIONAL\s\)'
+    return true
+  endif
+
+  var n = first_line
+  while n < 100 && n <= line("$")
+    # Forth comments and colon definitions
+    if getline(n) =~ '^[:(\\] '
+      return true
+    endif
+    n += 1
+  endwhile
+  return false
+enddef
+
+# Distinguish between Forth and Fortran
+export def FTf()
+  if exists("g:filetype_f")
+    exe "setf " .. g:filetype_f
+  elseif IsForth()
+    setf forth
+  else
+    setf fortran
+  endif
+enddef
+
 export def FTfrm()
   if exists("g:filetype_frm")
     exe "setf " .. g:filetype_frm
@@ -302,21 +333,13 @@ export def FTfrm()
   endif
 enddef
 
-# Distinguish between Forth and F#.
-# Provided by Doug Kearns.
+# Distinguish between Forth and F#
 export def FTfs()
   if exists("g:filetype_fs")
     exe "setf " .. g:filetype_fs
+  elseif IsForth()
+    setf forth
   else
-    var n = 1
-    while n < 100 && n <= line("$")
-      # Forth comments and colon definitions
-      if getline(n) =~ "^[:(\\\\] "
-        setf forth
-        return
-      endif
-      n += 1
-    endwhile
     setf fsharp
   endif
 enddef

runtime/doc/filetype.txt

@@ -146,6 +146,7 @@ variables can be used to overrule the filetype used for certain extensions:
 	*.cls		g:filetype_cls
 	*.csh		g:filetype_csh	|ft-csh-syntax|
 	*.dat		g:filetype_dat
+	*.f		g:filetype_f	|ft-forth-syntax|
 	*.frm		g:filetype_frm	|ft-form-syntax|
 	*.fs		g:filetype_fs	|ft-forth-syntax|
 	*.i		g:filetype_i	|ft-progress-syntax|

runtime/doc/syntax.txt

@@ -1579,9 +1579,10 @@ example, FORM files, use this in your startup vimrc: >
 
 FORTH						*forth.vim* *ft-forth-syntax*
 
-Files matching "*.fs" could be F# or Forth.  If the automatic detection
-doesn't work for you, or you don't edit F# at all, use this in your
-startup vimrc: >
+Files matching "*.f" could be Fortran or Forth and those matching "*.fs" could
+be F# or Forth.  If the automatic detection doesn't work for you, or you don't
+edit F# or Fortran at all, use this in your startup vimrc: >
+   :let filetype_f  = "forth"
    :let filetype_fs = "forth"
 
 

runtime/filetype.vim

@@ -724,16 +724,19 @@ au BufNewFile,BufRead auto.master		setf conf
 au BufNewFile,BufRead *.mas,*.master		setf master
 
 " Forth
-au BufNewFile,BufRead *.ft,*.fth		setf forth
+au BufNewFile,BufRead *.ft,*.fth,*.4th		setf forth
 
 " Reva Forth
 au BufNewFile,BufRead *.frt			setf reva
 
 " Fortran
 if has("fname_case")
-  au BufNewFile,BufRead *.F,*.FOR,*.FPP,*.FTN,*.F77,*.F90,*.F95,*.F03,*.F08	 setf fortran
+  au BufNewFile,BufRead *.F,*.FOR,*.FPP,*.FTN,*.F77,*.F90,*.F95,*.F03,*.F08	setf fortran
 endif
-au BufNewFile,BufRead   *.f,*.for,*.fortran,*.fpp,*.ftn,*.f77,*.f90,*.f95,*.f03,*.f08  setf fortran
+au BufNewFile,BufRead *.for,*.fortran,*.fpp,*.ftn,*.f77,*.f90,*.f95,*.f03,*.f08	setf fortran
+
+" Fortran or Forth
+au BufNewFile,BufRead *.f			call dist#ft#FTf()
 
 " Framescript
 au BufNewFile,BufRead *.fsl			setf framescript