mirror of https://github.com/texmacs/doc.git
95 lines
3.8 KiB
Tcl
95 lines
3.8 KiB
Tcl
<TeXmacs|1.0.7.17>
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<style|tmdoc>
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<\body>
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<tmdoc-title|Contextual overloading>
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For large software projects, it is important that different modules can be
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developed as independently as possible one from each other. Furthermore,
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fundamental modules often implement default behaviour which is to be
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overwritten in a more specialized module. In order to facilitate these two
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requirements, <TeXmacs> implements a system of <em|contextual overloading>.
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In order to get the main idea behind this system, consider the
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implementation of a given functionality, like hitting the return key.
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Depending on the context, different actions have to be undertaken: by
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default, we start a new paragraph; inside a table, we start a new row; etc.
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A naive implementation would check all possible cases in a routine
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<scm|kbd-enter> and call the corresponding routine. However, this makes it
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impossible to add a new case in a new module without modifying the module
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which defines <scm|kbd-enter>. By contrast, the system of contextual
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overloading allows the user to <em|conditionally> redefine the routine
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<scm|kbd-enter> several times in distinct modules.
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For instance, assume that we want to define a function <scm|hello> which
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inserts ``Hello'' by default, but ``<math|hello<around|(||)>>'' in mode
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math, while positioning the cursor between the brackets. Using contextual
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overloading, this may be done as follows:
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<\scm-code>
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(tm-define (hello) (insert "Hello"))
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(tm-define (hello) (:require (in-math?)) (insert-go-to "hello()" '(6)))
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</scm-code>
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\;
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The order in which routines are overloaded is important. <TeXmacs> first
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tries the latest (re)definition. If this definition does not satisfy the
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requirements (<scm|(in-math?)>, in our case), then it tries the before last
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(re)definition, and so on until an implementation is found which matches
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the requirements. For example, if we invert the two declarations in the
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above example, then the general unconditional definition of <scm|hello>
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will always prevail. If the two declarations are made inside different
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modules, then it is up to the user to ensure that the modules are loaded in
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an appropriate order.
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Inside a redefinition, it is also possible to access the former definition
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using the keyword <scm|former>. In particular, the code
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<\scm-code>
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(tm-define (hello)
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\ \ (if (in-math?) (insert-go-to "hello()" '(6)) (former)))
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</scm-code>
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is equivalent to the second declaration in our example.
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Contextual overloading generalizes more classical overloading on the types
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of the arguments, such as <name|C++> style polymorphism. Although one may
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overload on the types of the arguments, it is also possible to impose more
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general conditions on the arguments. For instance, one may sometimes wish
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to write the following kind of code:
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<\scm-code>
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(tm-define (my-replace what by)
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\ \ <em|default-implementation>)
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\;
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(tm-define (my-replace what by)
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\ \ (:require (== what by))
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\ \ (noop))
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</scm-code>
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Besides <scm|tm-define>, several other added language primitives support
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the contextual overloading mechanism. For instance, <scm|kbd-map> and
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<scm|menu-bind> support overloading on mode. The <scm|tm-define-macro> and
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<scm|tm-property> primitives are analoguous to <scm|tm-define>.
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<tmdoc-copyright|2005|Joris van der Hoeven>
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<tmdoc-license|Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation; with no
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Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled "GNU Free
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Documentation License".>
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</body>
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<initial|<\collection>
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</collection>> |