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I don't understand your example. Why is it necessary for with-file to me a macro? It seems like any language with lambdas could write it as a higher-order func

I don't understand your example. Why is it necessary for with-file to me a macro? It seems like any language with lambdas could write it as a higher-order function that asked for a consumer. How does having a with-file macro fix the "what if you forget it?" issue? It seems like you can forget with-file as easily as you can forget `using`.

pg314 11y ago
The with-x style macros can indeed be replaced by functions that take a closure. E.g. in Scheme you have the function (call-with-input-file string proc). The macro is slightly more concise. Contrast (with-input-file (x "x") (print (read x))) ; Common Lisp with (call-with-input-file "x" (lambda (x) (print (read x)))) ; Scheme
- chaoky 11y ago
  No runtime consing of closures either! In fact, in Common Lisp there is a style of writing call-with-foo that takes a closure and wrapping that with a macro with-foo. Since the macro abstracts that detail of having call-with, it is much more flexible. For example the macro could expand some stuff known at compile time and do some compile time computation too., instead of the closure consing function.
AKrumbach 11y ago
I feel pg314 did an adequate job answering your first question -- although I would have explained it that any meta-programming macros must be, by definition, higher-order functions. The only differences are giving the consumer function inline syntax and partial execution at compile time: both potent benefits. With-file might not be the very best example of "what if you forget", because it's so very simple. A more complex example -- where "forgetting" might be more than just a single function call -- would be handling semaphores and locks for threading. A well-written "with-lock" macro could ensure that these resources are obtained and released in a determined order (i.e. compute some sort of hierarchy for locks and enforce this when obtaining multiple locks).

pg314 11y ago

The with-x style macros can indeed be replaced by functions that take a closure. E.g. in Scheme you have the function (call-with-input-file string proc). The macro is slightly more concise. Contrast (with-input-file (x "x") (print (read x))) ; Common Lisp with (call-with-input-file "x" (lambda (x) (print (read x)))) ; Scheme

chaoky 11y ago
No runtime consing of closures either! In fact, in Common Lisp there is a style of writing call-with-foo that takes a closure and wrapping that with a macro with-foo. Since the macro abstracts that detail of having call-with, it is much more flexible. For example the macro could expand some stuff known at compile time and do some compile time computation too., instead of the closure consing function.

AKrumbach 11y ago

I feel pg314 did an adequate job answering your first question -- although I would have explained it that any meta-programming macros must be, by definition, higher-order functions. The only differences are giving the consumer function inline syntax and partial execution at compile time: both potent benefits. With-file might not be the very best example of "what if you forget", because it's so very simple. A more complex example -- where "forgetting" might be more than just a single function call -- would be handling semaphores and locks for threading. A well-written "with-lock" macro could ensure that these resources are obtained and released in a determined order (i.e. compute some sort of hierarchy for locks and enforce this when obtaining multiple locks).