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GNU Guile is an Interpreter / Virtual Machine for the Scheme Programming Language . It was first released in 1993.Blandy 1997, p. 102. Guile includes modularized extensions for POSIX system calls, APL Array functionality, and others, packaged as an Object "Strictly speaking, Guile is an object library, not an executable." Blandy 1997, p. 89. Library . "Libguile" allows the language to be embedded in other programs, and used as an interface for other languages which allow close integration"To encourage customization, Guile provides extensive interfaces, allowing C code to interact with the Scheme world. C code can freely create, access, and mutate Scheme objects; C Functions may call Scheme functions and vice versa; C code may add new types to the Scheme world and take advantage of Guile's garbage collection… Most of the standard Scheme procedures are implemented by C functions, visible to Guile clients; for example, applications can call the C function scm_cons, which is the underlying implementation of the Scheme procedures Cons ." Blandy 1997, pp. 94, 96..Guile is the "official" Extension Language of the GNU Project , although, as of 2006, it has been used for only a handful of major projects. Its name was coined in a Usenet discussion by Lee Thomas ."The name Guile was first suggested in a Usenet discussion by Lee Thomas." Guile Scheme 1995. The idea is that "the developer implements critical algorithms and data structures in C or C++ and exports the functions and types for use by interpreted code. The application becomes a library of primitives orchestrated by the interpreter, combining the efficiency of compiled code with the flexibility of interpretation."Blandy 1997, pp 87. This close interaction can come at a cost, however. Scheme requires implementations to optimize Tail Recursion because of Scheme's heavy use of recursion, but most techniques interfere with interoperation; Guile is forced to compromise and optimize tail calls within purely Scheme functions and programs, but to abandon tail recursion when C functions enter the picture.Blandy 1997, p. 99. Implementation of Call/cc , another requirement of the Scheme standard, is also unsatisfactory — to handle Continuation s with C involves copying the entire C Stack into the Heap ."Because Guile allows C functions and Scheme functions to call each other freely, a Guile continuation may include both C and Scheme stack frames. For simplicity, Guile's implementation of call/cc copies the entire C stack into the heap; invoking a continuation copies the stack back from the heap and uses the longjmp function to reactivate it. This implementation has a number of drawbacks…", Blandy 1997, p. 99. Garbage Collection , too, is not high-performance because C code could well have a Pointer to a Scheme object; to avoid loss of needed cons cells, Guile's garbage collector is a conservative one which can miss cells that could be reclaimed.Blandy 1997, pp. 99–100.SCHEME COMPLIANCE For much of its history, Guile did not strictly adhere to Scheme standards: The Guile version of Scheme differs from standard Scheme ( In recent releases of Guile, symbols are still case sensitive, but the empty list and Boolean false are now distinct. HISTORY Guile had its origin in a heated discussion (later dubbed "the Tcl from SIOD is still an important part of Guile. SIOD is still actively developed and freely available (search for "siod"). It has a very small footprint." Guile Scheme 1995.. One of the goals of Guile is to allow other languages to be translated into Scheme and thence into Portable Byte-code ; thus Guile would effectively be a language-neutral runtime environment. This goal has not been fulfilled yet, though various attempts have been made (a dialect of Scheme essentially differing only in its C-like syntax; a translation of Emacs Lisp ; a Tcl converter motivated by TkWWW; and something roughly resembling the Logo Programming Language ). Guile Scheme supports XML , XPath , and XSLT in the forms of SXML, SXPath and SXSLT, respectively. The S-expression -based XML processing is provided by guile-lib. Guile is supported by SLIB the portable Scheme library. PROGRAMS USING GUILE
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