Well. Not an expert so this is lacking nuance: So Dyalog back some years ago came up dfns. Traditional Dyalog functions are something like: RV←foo MyFun bar

Well. Not an expert so this is lacking nuance: So Dyalog back some years ago came up dfns. Traditional Dyalog functions are something like: RV←foo MyFun bar;x;y;z .... .... where RV is semi-optional the return value; foo and bar are the right and left arguements; and x y z are all local variables. Any variables declared in the function body are global. You can't nest functions like this. Dfns look like MyFun ← { ... } this comes with four variables automatically ⍺ ⍵ which are the right and left values and ⍺⍺ ⍵⍵ the right and left functions. you can nest dfns and variables declared inside are lexically scoped. Co-Dfns is a compiler (written in Dyalog) which adds parallel primitives into this existing dialect of Dyalog.