4.13 Dynamic Binding: parameterize
parameterize is used to have values that are “dynamically scoped”. You get a parameter with make-parameter. The parameter itself behaves as a function: call it with no inputs and you get its value, call it with one value and it will set the value. The settings that are adjusted by a parameterize form are called parameters. For example:
The term “parameter” is sometimes used to refer to the arguments of a function, but “parameter” in PLT Scheme has the more specific meaning described here.
Examples: |
| > (define p (make-parameter "blah")) |
| > (p) |
"blah" |
| > (p "meh") |
| > (p) |
"meh" |
Many functions (including many primitive ones) use parameters as a way to customize their behavior. For example printf will print text using the port that is the value of the current-output-port parameter. Now, say that you have some function that prints something:
Example: |
| > (define (foo x) (printf "the value of x is ~s\n")) |
You usually call this function and see something printed on the screen – but in some cases you want to use it to print something to a file or whatever. You could do this:
Example: | |||||
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One problem with this is that it is tedious to do – but that’s easily solved with a macro. (In fact, PLT still has a construct that does that in some languages: fluid-let.) But there are more problems here: what happens if the call to foo results in a runtime error? This might leave the system in a bad state, where all output goes to your port (and you won’t even see a problem, since it won’t print anything). A solution for that (which fluid-let uses too) is to protect the saving/restoring of the parameter with dynamic-wind, which makes sure that if there’s an error (and more, if you know about continuations) then the value is still restored.
So the question is what’s the point of having parameters instead of just using globals and fluid-let? There are two more problems that you cannot solve with just globals. One is what happens when you have multiple threads – in this case, setting the value temporarily will affect other threads, which may still want to print to the standard output. Parameters solve this by having a specific value per-thread. What happens is that each thread “inherits” the value from the thread that created it, and changes in one thread are visible only in that thread.
The other problem is more subtle. Say that you have a parameter with a numeric value, and you want to do the following:
Example: | |||
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In Scheme, “tail calls” are important – they are the basic tool for creating loops and much more. parameterize does some magic that allows it to change the parameter value temporarily but still preserve these tail calls. For example, in the above case, you will get an infinite loop, rather than get a stack overflow error – what happens is that each of these parameterize expressions can somehow detect when there’s an earlier parameterize that no longer needs to do its cleanup.
Finally, parameterize actually uses two important parts of PLT to do its job: it uses thread cells to implement per-thread values, and it uses continuation marks to be able to preserve tail-calls. Each of these features is useful in itself.
(parameterize ([parameter-expr value-expr] ...) body ...+)
The result of a parameterize form is the result of the last body expression. While the body expressions are evaluated, the parameter produced by each parameter-expr is set to the result of the corresponding value-expr.
Many parameters are built in. For example, the error-print-width parameter controls how many characters of a value are printed in an error message (in case the printed form of the value is very large):
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car: expects argument of type <pair>; given 1000000... | ||
| ||
car: expects argument of type <pair>; given 10... |
The error-print-width parameter acts like a kind of default argument to the function that formats error messages. This parameter-based argument can be configured far from the actual call to the error-formatting function, which in this case is called deep within the implementation of car.
The parameterize form adjusts the value of a parameter only while evaluating its body expressions. After the body produces a value, the parameter reverts to its previous value. If control escapes from the body due to an exception, as in the above example, then the parameter value is restored in that case, too. Finally, parameter values are thread-specific, so that multiple threads do not interfere with each others’ settings.
Use make-parameter to create a new parameter that works with parameterize. The argument to make-parameter is the value of the parameter when it is not otherwise set by parameterize. To access the current value of the parameter, call it like a function.
| > (define favorite-flavor (make-parameter 'chocolate)) | ||
| > (favorite-flavor) | ||
chocolate | ||
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((scoop of strawberry) (scoop of strawberry) (scoop of strawberry)) | ||
| > (ice-cream 3) | ||
((scoop of chocolate) (scoop of chocolate) (scoop of chocolate)) |