I recall there being some similar attempts presented on this list but
I couldn't find working example so I hacked up the following. It has
few rough edges and wasn't tested thoroughly but appears to work.
]USERCMD ]EDIT EDIT⍙ 1
∇S EDIT⍙ Q;FN;FD
⍝ Edit a function or operator in external editor.
⍎(2>≡S)/"']EDIT NAME' ⋄ →0"
S←(⎕IO+1)⊃S
⍎(~(⎕NC S)∊0 3 4)/"'Bad name ',S ⋄ →0"
FN←'/tmp/',S,'.',⍕↑⎕AI
FD←'w'⎕FIO[3] 18⎕CR FN
⍎(0>FD)/"'Can''t open file ',FN ⋄ →0"
Q←(⎕IO+0≠⎕NC S)⊃(⍉⍪S)(⎕CR S)
Q←{⍵↓⍨-+/∧\⌽' '=⍵}¨⊂[⎕IO+1]Q
Q←∊Q,¨⊂"\n"
⊣ Q ⎕FIO[23]FD
⊣ ⎕FIO[4]FD
EDIT⍙OPEN FN
Q←⎕FIO[26] 18⎕CR FN
⊣ ⎕FIO[19] 18⎕CR FN
⍎(~' '≡↑0⍴Q)/"'Filed to read ',FN ⋄ →0"
⍎(0=⍴Q)/"'Empty' ⋄ →0"
⍎(0=⍴Q←19⎕CR Q)/"'Encoding error' ⋄ →0"
⎕FX (~"\n"=Q)⊂Q
∇
⍝ External editor to use; defaults to $EDITOR or vi
EDIT⍙EDITOR←,⎕ENV'EDITOR'
EDIT⍙EDITOR←(1+EDIT⍙EDITOR⍳⊂'EDITOR')⊃EDIT⍙EDITOR,⍬'vi'
∇EDIT⍙OPEN FILE;STTY
⍝ Open a file in external editor.
((''''=FILE)/FILE)←⊂"'\''"
FILE←'''',∊FILE,''''
STTY←⎕IO⊃⍎')HOST stty -g'
⊣⍎')HOST >/dev/tty stty sane'
⊣⍎')HOST >/dev/tty ',EDIT⍙EDITOR,' ',FILE
⊣⍎')HOST >/dev/tty stty ',STTY
∇
