lazy evaluation - Why isn't promise a data type in Scheme? -

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the object returned delay in scheme "a promise", promises not considered type (so there no promise? procedure, , it's not listed type in r5rs or r6rs).

is there strong reson why so? seem quite natural me (if (promise? x) (force x) x), example. (and see implementations let me force non-promises, , others not). also, if can store in variale , pass around, feel should have type.

there can't strong reason, since mit/gnu scheme, defines promise? function.


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