Node:Fluids, Previous:Threads, Up:Scheduling
Fluids are objects to store values in. They have a few properties which make them useful in certain situations: Fluids can have one value per dynamic root (see Dynamic Roots), so that changes to the value in a fluid are only visible in the same dynamic root. Since threads are executed in separate dynamic roots, fluids can be used for thread local storage (see Threads).
Fluids can be used to simulate dynamically scoped variables. These are
used in several (especially in older) dialects of lisp, such as in Emacs
Lisp, and they work a bit like global variables in that they can be
modified by the caller of a procedure, and the called procedure will see
the changes. With lexically scoped variables--which are normally used
in Scheme--this cannot happen. See the description of
with-fluids* below for details.
New fluids are created with make-fluid and fluid? is used
for testing whether an object is actually a fluid.
| make-fluid | Scheme Procedure |
| scm_make_fluid () | C Function |
| Return a newly created fluid. Fluids are objects of a certain type (a smob) that can hold one SCM value per dynamic root. That is, modifications to this value are only visible to code that executes within the same dynamic root as the modifying code. When a new dynamic root is constructed, it inherits the values from its parent. Because each thread executes in its own dynamic root, you can use fluids for thread local storage. |
| fluid? obj | Scheme Procedure |
| scm_fluid_p (obj) | C Function |
Return #t iff obj is a fluid; otherwise, return
#f.
|
The values stored in a fluid can be accessed with fluid-ref and
fluid-set!.
| fluid-ref fluid | Scheme Procedure |
| scm_fluid_ref (fluid) | C Function |
Return the value associated with fluid in the current
dynamic root. If fluid has not been set, then return
#f.
|
| fluid-set! fluid value | Scheme Procedure |
| scm_fluid_set_x (fluid, value) | C Function |
| Set the value associated with fluid in the current dynamic root. |
with-fluids* temporarily changes the values of one or more fluids,
so that the given procedure and each procedure called by it access the
given values. After the procedure returns, the old values are restored.
| with-fluids* fluids values thunk | Scheme Procedure |
| scm_with_fluids (fluids, values, thunk) | C Function |
| Set fluids to values temporary, and call thunk. fluids must be a list of fluids and values must be the same number of their values to be applied. Each substitution is done one after another. thunk must be a procedure with no argument. |