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

"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