AppleScript

gistfile1.applescript

-- 1. TextEdit got an error: Can’t get POSIX file "..."
set p to "/path/to/file.txt"

tell application "TextEdit"
  open file (POSIX file p)
end tell

-- 2. This works
tell application "TextEdit"
  -- use literal
  open file (POSIX file "/path/to/file.txt")
end tell

-- 3. This works
set p to "/path/to/file.txt"

tell application "TextEdit"
  open file (p as POSIX file)
end tell

-- 4. This works
set p to "/path/to/file.txt"
set q to POSIX file p

tell application "TextEdit"
  open file q
end tell

tell application "TextEdit" to set resultDocument to open "/Users/me/Desktop/file.txt"
-- or
tell application "TextEdit" to set resultDocument to open alias "HD:Users:me:Desktop:file.txt"
-- or
tell application "TextEdit" to set resultDocument to open "HD:Users:me:Desktop:file.txt"
-- or
tell application "Finder" to set myFile to document file "file" of folder "Desktop" of folder "me" of folder "Users" of disk "HD"
tell application "TextEdit" to set resultDocument to open myFile

"Path to" will help with hardcoding.

set filePath to POSIX path of (path to desktop as text) & "file.txt"
tell application "TextEdit" to set resultDocument to open filePath