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.
Jay On Fri, Nov 7, 2014 at 4:08 PM, Matthew Butterick <m...@mbtype.com> wrote: > Since `hash-ref!` includes `or`-like functionality, is there a reason it's > not implemented with short-circuit logic similar to `or`? > > In other words, the function `hash-ref!` is invoked thus: > > (hash-ref! hashtable key new-value-if-needed) > > What I notice is that if you put an expression in the new-value position, > like so: > > (hash-ref! hashtable key (expression-that-produces-value)) > > Then this expression will always be evaluated before the function is called. > > But this seems wasteful, since if 'key' exists in 'hashtable', the expression > won't be needed. > > `hash-ref!` will also take a thunk, so you can manually delay evaluation of > the value expression by wrapping it in λ: > > (hash-ref! hashtable key (λ () (expression-that-produces-value))) > > But I'm curious why delayed evaluation isn't the default behavior. > ____________________ > Racket Users list: > http://lists.racket-lang.org/users -- Jay McCarthy http://jeapostrophe.github.io "Wherefore, be not weary in well-doing, for ye are laying the foundation of a great work. And out of small things proceedeth that which is great." - D&C 64:33 ____________________ Racket Users list: http://lists.racket-lang.org/users
