2 @c This is part of the GNU Guile Reference Manual.
3 @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010
4 @c Free Software Foundation, Inc.
5 @c See the file guile.texi for copying conditions.
7 @node Binding Constructs
8 @section Definitions and Variable Bindings
10 Scheme supports the definition of variables in different contexts.
11 Variables can be defined at the top level, so that they are visible in
12 the entire program, and variables can be defined locally to procedures
13 and expressions. This is important for modularity and data abstraction.
16 * Top Level:: Top level variable definitions.
17 * Local Bindings:: Local variable bindings.
18 * Internal Definitions:: Internal definitions.
19 * Binding Reflection:: Querying variable bindings.
24 @subsection Top Level Variable Definitions
26 @cindex variable definition
28 At the top level of a program (i.e., not nested within any other
29 expression), a definition of the form
32 (define a @var{value})
36 defines a variable called @code{a} and sets it to the value @var{value}.
38 If the variable already exists in the current module, because it has
39 already been created by a previous @code{define} expression with the
40 same name, its value is simply changed to the new @var{value}. In this
41 case, then, the above form is completely equivalent to
48 This equivalence means that @code{define} can be used interchangeably
49 with @code{set!} to change the value of variables at the top level of
50 the REPL or a Scheme source file. It is useful during interactive
51 development when reloading a Scheme file that you have modified, because
52 it allows the @code{define} expressions in that file to work as expected
53 both the first time that the file is loaded and on subsequent occasions.
55 Note, though, that @code{define} and @code{set!} are not always
56 equivalent. For example, a @code{set!} is not allowed if the named
57 variable does not already exist, and the two expressions can behave
58 differently in the case where there are imported variables visible from
61 @deffn {Scheme Syntax} define name value
62 Create a top level variable named @var{name} with value @var{value}.
63 If the named variable already exists, just change its value. The return
64 value of a @code{define} expression is unspecified.
67 The C API equivalents of @code{define} are @code{scm_define} and
68 @code{scm_c_define}, which differ from each other in whether the
69 variable name is specified as a @code{SCM} symbol or as a
70 null-terminated C string.
72 @deffn {C Function} scm_define (sym, value)
73 @deffnx {C Function} scm_c_define (const char *name, value)
74 C equivalents of @code{define}, with variable name specified either by
75 @var{sym}, a symbol, or by @var{name}, a null-terminated C string. Both
76 variants return the new or preexisting variable object.
79 @code{define} (when it occurs at top level), @code{scm_define} and
80 @code{scm_c_define} all create or set the value of a variable in the top
81 level environment of the current module. If there was not already a
82 variable with the specified name belonging to the current module, but a
83 similarly named variable from another module was visible through having
84 been imported, the newly created variable in the current module will
85 shadow the imported variable, such that the imported variable is no
88 Attention: Scheme definitions inside local binding constructs
89 (@pxref{Local Bindings}) act differently (@pxref{Internal Definitions}).
93 @subsection Local Variable Bindings
95 @cindex local bindings
96 @cindex local variables
98 As opposed to definitions at the top level, which creates bindings that
99 are visible to all code in a module, it is also possible to define
100 variables which are only visible in a well-defined part of the program.
101 Normally, this part of a program will be a procedure or a subexpression
104 With the constructs for local binding (@code{let}, @code{let*},
105 @code{letrec}, and @code{letrec*}), the Scheme language has a block
106 structure like most other programming languages since the days of
107 @sc{Algol 60}. Readers familiar to languages like C or Java should
108 already be used to this concept, but the family of @code{let}
109 expressions has a few properties which are well worth knowing.
111 The most basic local binding construct is @code{let}.
113 @deffn syntax let bindings body
114 @var{bindings} has the form
117 ((@var{variable1} @var{init1}) @dots{})
120 that is zero or more two-element lists of a variable and an arbitrary
121 expression each. All @var{variable} names must be distinct.
123 A @code{let} expression is evaluated as follows.
127 All @var{init} expressions are evaluated.
130 New storage is allocated for the @var{variables}.
133 The values of the @var{init} expressions are stored into the variables.
136 The expressions in @var{body} are evaluated in order, and the value of
137 the last expression is returned as the value of the @code{let}
141 The @var{init} expressions are not allowed to refer to any of the
145 The other binding contructs are variations on the same theme: making new
146 values, binding them to variables, and executing a body in that new,
147 extended lexical context.
149 @deffn syntax let* bindings body
150 Similar to @code{let}, but the variable bindings are performed
151 sequentially, that means that all @var{init} expression are allowed to
152 use the variables defined on their left in the binding list.
154 A @code{let*} expression can always be expressed with nested @code{let}
167 @deffn syntax letrec bindings body
168 Similar to @code{let}, but it is possible to refer to the @var{variable}
169 from lambda expression created in any of the @var{inits}. That is,
170 procedures created in the @var{init} expression can recursively refer to
171 the defined variables.
174 (letrec ((even? (lambda (n)
187 Note that while the @var{init} expressions may refer to the new
188 variables, they may not access their values. For example, making the
189 @code{even?} function above creates a closure (@pxref{About Closure})
190 referencing the @code{odd?} variable. But @code{odd?} can't be called
191 until after execution has entered the body.
194 @deffn syntax letrec* bindings body
195 Similar to @code{letrec}, except the @var{init} expressions are bound to
196 their variables in order.
198 @code{letrec*} thus relaxes the letrec restriction, in that later
199 @var{init} expressions may refer to the values of previously bound
206 @result{} ;; Error: unbound variable: a
215 There is also an alternative form of the @code{let} form, which is used
216 for expressing iteration. Because of the use as a looping construct,
217 this form (the @dfn{named let}) is documented in the section about
218 iteration (@pxref{while do, Iteration})
220 @node Internal Definitions
221 @subsection Internal definitions
223 @c FIXME::martin: Review me!
225 A @code{define} form which appears inside the body of a @code{lambda},
226 @code{let}, @code{let*}, @code{letrec}, @code{letrec*} or equivalent
227 expression is called an @dfn{internal definition}. An internal
228 definition differs from a top level definition (@pxref{Top Level}),
229 because the definition is only visible inside the complete body of the
230 enclosing form. Let us examine the following example.
233 (let ((frumble "froz"))
234 (define banana (lambda () (apple 'peach)))
235 (define apple (lambda (x) x))
241 Here the enclosing form is a @code{let}, so the @code{define}s in the
242 @code{let}-body are internal definitions. Because the scope of the
243 internal definitions is the @strong{complete} body of the
244 @code{let}-expression, the @code{lambda}-expression which gets bound to
245 the variable @code{banana} may refer to the variable @code{apple}, even
246 though its definition appears lexically @emph{after} the definition of
247 @code{banana}. This is because a sequence of internal definition acts
248 as if it were a @code{letrec*} expression.
262 (letrec* ((a 1) (b 2))
266 Internal definitions are only allowed at the beginning of the body of an
267 enclosing expression. They may not be mixed with other expressions.
269 Another noteworthy difference to top level definitions is that within
270 one group of internal definitions all variable names must be distinct.
271 That means where on the top level a second define for a given variable
272 acts like a @code{set!}, an exception is thrown for internal definitions
273 with duplicate bindings.
275 As a historical note, it used to be that internal bindings were expanded
276 in terms of @code{letrec}, not @code{letrec*}. This was the situation
277 for the R5RS report and before. However with the R6RS, it was recognized
278 that sequential definition was a more intuitive expansion, as in the
289 Guile decided to follow the R6RS in this regard, and now expands
290 internal definitions using @code{letrec*}.
293 @node Binding Reflection
294 @subsection Querying variable bindings
296 Guile provides a procedure for checking whether a symbol is bound in the
297 top level environment.
299 @deffn {Scheme Procedure} defined? sym [module]
300 @deffnx {C Function} scm_defined_p (sym, module)
301 Return @code{#t} if @var{sym} is defined in the module @var{module} or
302 the current module when @var{module} is not specified; otherwise return
308 @c TeX-master: "guile.texi"