Making it the default behavior would require it to be a macro instead of an
ordinary function, so you’d lose the ability to pass it around as a
function. Given that implementing a short circuit macro on top of hash-ref!
is as simple as (define-syntax-rule (hash-ref!! table key expr) (hash-ref!
tabl
The simplest, boring, answer is that short-circuiting requires that
hash-ref! be a macro, which makes it difficult to use in first-class
ways, like storing it inside of a list. There are many other functions
like this and the pattern of accepting a value OR a thunk is pretty
common, for this reason
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