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set thePath to "~/Downloads" -- $HOME-based POSIX Path.
set myAlias1 to makeAliasFromPathString(thePath)
set thePath to "/Users/chris/Downloads" -- POSIX Path
set myAlias2 to makeAliasFromPathString(thePath)
set thePath to (path to downloads folder as text) -- HFS Path
set myAlias3 to makeAliasFromPathString(thePath)
on makeAliasFromPathString(thePath) -- Input: POSIX-Path, Tilde-Path, or HFS-Path
tell application "System Events" to set thePath to POSIX path of disk item thePath
return (POSIX file thePath) as alias
end makeAliasFromPathString
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