Testing the usual suite of Schemes to see whether promises returned by delay are disjoint from procedures.
Disjoint: Racket, Gauche, MIT, Gambit, Chicken, Guile, Kawa, Chibi, SCM, STklos, SigScheme, TinyScheme, RScheme, Dream, BDC, XLisp, Elk, UMB, VX, SXM, Sizzle, Spark, Sagittarius (note that in TinyScheme and Dream promises can be invoked)
Not disjoint: Bigloo, Scheme48/scsh, SISC, Chez, Vicare, Larceny, Ypsilon, Mosh, IronScheme, NexJ, Scheme 9, Schemik, Inlab, Owl Lisp
delay undefined: KSi, Shoe, S7, Rep, Llava, FemtoLisp, Dfsch
Buggy implementation: Oaklisp
Note: In Owl Lisp, repeated forces re-evaluate the delayed expression, because Owl Lisp is immutable.
See also: Force non-promise.