* | Expressions | | Value constructors expressions |
The page Types and patterns presents
the different kind of values: scalar constant (integers, characters, atoms),
structured values (pairs, records, sequences, XML elements),
and functional values (abstractions). Value themselves are
expressions, and the value constructors for structured values
operate also on expressions.
This page presents the other kinds of expressions in the language.
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| Pattern matching |
A fundamental operation in CDuce is pattern matching:
match e with
| p1 -> e1
...
| pn -> en
The first vertical bar | can be omitted.
The semantics is to try to match the result of the evaluation
of e successively with each pattern
pi. The first matching pattern triggers
the corresponding expression in the right hand side,
which can use the variables bound by the pattern.
Note that a first match policy, as for the disjunction patterns.
The static type system ensures that the pattern matching is exhaustive:
the type computed for e must be
a subtype of the union of the types accepted by all the patterns.
Local definition is a lighter notation for a pattern matching with
a single branch:
is equivalent to:
Note that the pattern p need not be a simple
capture variable.
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| Functions | Abstraction
The general form for a function expression is:
fun f (t1 -> s1; ...; tn -> sn)
| p1 -> e1
...
| pn -> en
The first line is the interface of the function,
and the remaining is the body, which is
a form of pattern matching (the first vertical bar | can
thus be omitted).
The identifier f is optional; it is useful
to define a recursive function (the body of the function can
use this identifier to refer to the function itself).
The interface of the function specifies some constraints on the
behavior of the function. Namely, when the function
receive an argument of type, say ti, the result
(if any) must be of type si. The type system
ensures this property by type-checking the body once for each constraint.
The function operate by pattern-matching the argument (which is a
value) exactly as for standard pattern matching. Actually, it
is always possible to add a line x -> match x with
between the interface and the body without changing the semantics.
When there is a single constraint in the interface, there is
an alternative notation, which is lighter for several arguments
(that is, when the argument is a tuple):
fun f (p1 : t1, ..., pn : tn) : s = e
(note the blank spaces around the colons which are mandatory when the
pattern is a variable
[1]) which is strictly
equivalent to:
fun f ((t1,...,tn) -> s) (p1,...,pn) -> e
It is also possible to define currified functions with this syntax:
fun f (p1 : t1, ..., pn : tn) (q1 : s1, ..., qm : sm) ... : s = e
which is strictly
equivalent to:
fun f ((t1,...,tn) -> (s1,...,sm) -> ... -> s)
(p1,...,pn) ->
fun ((s1,...,sm) -> ... -> s)
(q1,...,qm) ->
...
e
The standard notation for local binding a function is:
let f = fun g (...) ... in ...
Here, f is the "external" name for the function,
and g is the "internal" name (used when the function
needs to call itself recursively, for instance). When the two names coincide
(or when you don't need an internal name), there are lighter
notations:
let fun f (...) ... in ...
let f (...) ... in ...
Application
The only way
to use a function is ultimately to apply it to an argument. The notation
is simply a juxtaposition of the function and its argument.
E.g.:
(fun f (x : Int) : Int = x + 1) 10
evaluates to 11. The static type system ensures that
applications cannot fail.
Note that even if there is no functional "pattern" in CDuce,
it is possible to use in a pattern a type constraint
with a functional type, as in:
fun (Any -> Int)
| f & (Int -> Int) -> f 5
| x & Int -> x
| _ -> 0
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| Exceptions |
The following construction raises an exception:
The result of the evaluation of e is the
argument of the exception.
It is possible to catch an exception with an exception handler:
try e with
| p1 -> e1
...
| pn -> en
Whenever the evaluation of e raises an exception,
the handler tries to match the argument of the exception with
the patterns (following a first-match policy). If no pattern matches,
the exception is propagated.
Note that contrary to ML, there is no exception name: the only
information carried by the exception is its argument. Consequently,
it is the responsibility of the programmer to put enough information
in the argument to recognize the correct exceptions. Note also
that a branch (`A,x) -> e in an exception
handler gives no static information about the capture variable
x (its type is Any).
Note:
it is possible that the support for exceptions will change in the future
to match ML-like named exceptions.
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| Record operators |
There are three kinds of operators on records:
-
Field projection:
where
l is the name of a label which must be
present in the result of the evaluation of e.
This construction is equivalent to: match e with
{ l = x } -> x. It is necessary to put
whitespace between the expression and the dot
when the expression is an identifier.
-
Record concatenation:
The two expressions must evaluate to records, which
are merged together. If both have a field with the same
name, the one on the right have precedence. Note
that the operator + is overloaded: it also operates
on integers.
-
Field suppression:
deletes the field l in the record resulting from
the evaluation of e whenever it is present.
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| Arithmetic operators |
Binary arithmetic operators on integers:
+,-,*,div,mod. Note that / is used
for projection and not for division.
The operator +,- and * are typed
using simple interval arithmetic. The operators div
and mod produce a warning at compile type if
the type of there second argument include the integer 0.
The type Float represents floating point numbers.
An operator float_of: String -> Float is provided
to create values of this type. Currently, no other operator
are provided for this type (but you can use OCaml functions
to work on floats).
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| Generic comparisons, if-then-else |
Binary comparison operators (returns booleans):
=,<<,<=,>>,>=. Note that <
is used for XML elements and is this not available for comparison.
The semantics of the comparison is not specified when
the values contain functions. Otherwise, the comparison
gives a total ordering on CDuce values. The result type
for all the comparison operators is Bool, except
for equality when the arguments are known statically to be different
(their types are disjoint); in this case, the result type
is the singleton `false.
The if-then-else construction is standard:
and is equivalent to:
match e1 with `true -> e2 | `false -> e3
Note that the else-clause is mandatory.
The infix operators || and &&
denote respectively the logical or and the logical and. The prefix
operator not denotes the logical negation.
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| Upward coercions |
It is possible to "forget" that an expression has a precise type,
and give it a super-type:
The type of this expression if t, and
e must provably have this type (it can have a
subtype). This "upward coercion" can be combined with the local let
binding:
which is equivalent to:
Note that the upward coercion allows earlier detection of type errors,
better localization in the program, and more informative messages.
CDuce also have a dynamic type-check construction:
(e :? t)
let p :? t = e in ...
If the value resulting from the evaluation of e
does not have type t, an exception
whose argument (of type Latin1) explains the reason
of the mismatch is raised.
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| Sequences |
The concatenation operator is written @. There
is also a flatten operator which takes a sequence of
sequences and returns their concatenation.
There are two built-in constructions to iterate over a sequence.
Both have a very precise typing which takes into account
the position of elements in the input sequence as given by
its static type. The map construction is:
map e with
| p1 -> e1
...
| pn -> en
Note the syntactic similarity with pattern matching. Actually,
map is a pattern matching form,
where the branches are applied in turn to each element of the
input sequence (the result of the evaluation of e).
The semantics is to return a sequence of the same length, where
each element in the input sequence is replaced by the result of
the matching branch.
Contrary to map, the transform construction
can return a sequence of a different length. This is achieved
by letting each branch return a sequence instead of a single
element. The syntax is:
transform e with
| p1 -> e1
...
| pn -> en
There is always an implicit default branch _ -> []
at then end of transform, which means that
unmatched elements of the input sequence are simply discarded.
Note that map can be simulated by transform
by replacing each expression ei with
[ ei ].
Conversely, transform can be simulated by
map by using the flatten operator.
Indeed, we can rewrite transform e with ...
as flatten (map e with ... | _ -> []).
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| XML-specific constructions | Loading XML documents
The load_xml: Latin1 -> AnyXml built-in function parses
an XML document on the local
file system. The argument is the filename.
The result type AnyXml is defined as:
type AnyXml = <(Atom) (Record)>[ (AnyXml|Char)* ]
If the support for netclient or curl is available, it is also
possible to fetch an XML file from an URL, e.g.:
load_xml "http://...". A special scheme string:
is always supported: the string following the scheme is parsed as it is.
There is also a load_html: Latin1 -> [Any*] built-in
function to parse in a
permissive way HTML documents.
Pretty-printing XML documents
Two built-in functions can be used to produce a string from an XML document:
print_xml: Any -> Latin1
print_xml_utf8: Any -> String
They fail if the argument is not an XML document (this isn't checked
statically). The first operator
print_xml prepares the document to be dumped to
a ISO-8859-1 encoded XML file: Unicode characters outside Latin1
are escaped accordingly, and the operator fails if the document
contains tag or attribute names which cannot be represented
in ISO-8859-1. The second operator print_xml_utf8
always succeed but produces a string suitable for being dumped
in an UTF-8 encoded file. See the variants of the
dump_to_file operator
in the section on Input/output.
In both cases, the resulting string does not contain
the XML prefix "<?xml ...>".
Projection
The projection takes a sequence of XML elements and returns
the concatenation of all their children with a given type.
The syntax is:
which is equivalent to:
transform e with <_>[ (x::t | _)* ] -> x
For instance, the expression
[ <a>[ <x>"A" <y>"B" ] <b>[ <y>"C" <x>"D"] ] / <x>_
evaluates to
[ <x>"A" <x>"D" ]
.
There is another form of projection to extract attributes:
which is equivalent to:
transform e with <_ l=l>_ -> l
The dot notation can also be used to extract the value of the
attribute for one XML element:
# <a x=3>[].x;;
- : 3 = 3
Iteration over XML trees
Another XML-specific construction is xtransform
which is a generalization of transform to XML trees:
xtransform e with
| p1 -> e1
...
| pn -> en
Here, when an XML elements in the input sequence is not matched
by a pattern, the element is copied except that the transformation
is applied recursively to its content. Elements in the input sequence
which are not matched and are not XML elements are copied verbatim.
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| Unicode Strings |
Strings are nothing but sequences of characters, but in view of their
importance when dealing with XML we introduced the standard double
quote notation. So [ 'F' 'r' 'a' 'n' 'ç' 'e' ] can be
written as "Françe". In double quote all the
values of type Char can be used: so besides Unicode chars we
can also double-quote codepoint-defined characters (\xh;
\d; where h and d are
hexadecimal and decimal integers respectively), and backslash-escaped
characters (\t tab, \n newline,
\r return, \\ backslash). Instead we
cannot use character expressions that are not values. For instance, for
characters there is the built-in function char_of_int : Int
-> Char which returns the character corresponding to the given
Unicode codepoint (or raises an exception for a non-existent
codepoint), and this can only be used with the regular sequence
notation, thus "Françe", "Fran"@[(char_of_int
231)]@"e", and "Fran\231;e" are equivalent expressions.
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| Converting to and from string | Pretty-printing a value
The built-in function string_of: Any -> Latin1
converts any value to a string,
using the same pretty-printing function as the CDuce interpreter itself.
Creating and decomposing atoms from strings
The built-in functions split_atom: Atom ->
(String,String) and make_atom: (String,String) ->
Atom converts between atoms and pair of strings
(namespace,local name).
Creating integers from strings
The operator int_of converts a string to an integer. The string
is read in decimal (by default) or in hexadecimal (if it begins with
0x or 0X), octal (if it begins with 0o
or 0O), or binary (if it begins with 0b or
0B). It fails if the string is not a decimal representation of an
integer or if in the case of hexadecimal, octal, and binary representation the integer cannot be contained in 64 bits. There is a type-checking warning when the argument cannot be proved
to be of type [ '-'? '0'--'9'+ ] | ['-'? 'O'('b'|'B') '0'--'1'+ ] |
['-'? 'O'('o'|'O') '0'--'7'+ ] | ['-'? 'O'('x'|'X')
('0'--'9'|'a'--'f'|'A'--'F')+] .
Creating strings from integers
Besides the built-in function string_of: Any -> Latin1, it is
also possible to create characters, hence strings, from their codepoints:
either by enclosing their code within a backslash (\x for
hexadecimal code) and a semicolon, or by applying the built-in function
char_of_int : Int -> Char.
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| Input-output | Displaying a string
To print a string to standard output, you can use one of the built-in
function print: Latin1 -> [] or
print_utf8: String -> [].
Loading files
There are two built-in functions available to load a file into a CDuce string:
load_file: Latin1 -> Latin1
load_file_utf8: Latin1 -> String
The first one loads an ISO-8859-1 encoded file,
whereas the second
one loads a UTF-8 encoded file.
If the support for netclient or curl is available, it is also
possible to fetch a file from an URL, e.g.:
load_file "http://...".
Dumping to files
There are two operators available to dump a CDuce string to a file:
dump_to_file e1 e2
dump_to_file_utf8 e1 e2
The first one creates an ISO-8859-1 encoded file (it fails
when the CDuce string contains non Latin1 characters), whereas the second
one creates a UTF-8 encoded file. In both cases, the first
argument is the filename and the second one is the string to dump.
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| System | Running external commands
The predefined function system executes
an external command (passed to /bin/sh)
and returns its standard output and standard error
channels and its exit code. The type for system
is:
Latin1 -> { stdout = Latin1; stderr = Latin1;
status = (`exited,Int) | (`stopped,Int) | (`signaled,Int) |}
Terminating the program
The predefined function exit: 0--255 -> Empty terminates
the current process. The argument is the exit code.
Accessing the environment
The built-in function getenv: Latin1 -> Latin1
queries the system environment for an environment variable.
If the argument does not refer to an existing variable,
the function raises the exception `Not_found.
Command line arguments
The built-in function argv: [] -> [ String* ] returns
the sequence of command line arguments given to the current program.
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| Namespaces |
It is possible in expression position to define a local
prefix-namespace binding or to set a local default namespace.
namespace p = "..." in e
namespace "..." in e
See XML Namespaces for more details.
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| Imperative features |
The construction ref T e is used to build a
reference initialized with the result of the expression
e; later, the reference can receive any value
of type T. The reference is actually a value of type
{ get = [] -> T ; set = T -> [] }.
Two syntactic sugar constructions are provided to facilitate
the use of references:
!e === e.get [] Dereferencing
e1 := e2 === e1.set e2 Assignment
An expression of type [] is often considered
as a command and followed by another expression. The sequencing
operator gives a syntax for that:
e1 ; e2 === let [] = e1 in e2 Sequencing
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| Queries |
CDuce is endowed with a select_from_where syntax to perform some SQL-like queries. The general form of select expressions is
select e from
p1 in e1,
p2 in e2,
:
pn in en
where c
where e is an expression, c
a boolean expression, the pi's are patterns, and the
ei's are sequence expressions.
It works exactly as a standard SQL select expression, with the difference that
relations (that is sequences of tuples) after the in keyword can
here be generic sequences, and before the in generic patterns
instead of just capture variables can be used. So the result is the sequence of
all values obtained by calculating e in the sequence of
environments in which the free variables of e are bounded by
iteratively matching each pattern pi with every element of the
sequence ei, provided that the condition c is
satisfied. In other words, the first element of the result is obtained by
calculating e in the environment obtained by matching
p1 against the first element of e1,
p2 against the first element of e2, ... , and
pn against the first element of en; the
second element of the result is obtained by calculating e in
the environment obtained by matching p1 against the first
element of e1, p2 against the first element of
e2, ..., and pn against the second
element of en, ... ; and so on.
Formally, the semantics of the select expression above is defined as:
transform e1 with p1 ->
transform e2 with p2 ->
...
transform en with pn ->
if c then [e] else []
A select expression works like a set of nested
transform expressions. The advantage of using select rather than
transform is that queries are automatically optimized by applying classical
logic SQL optimization techniques (this automatic optimization can be
disabled).
The built-in optimizer is free to move boolean conditions around to
evaluate them as soon as possible. A warning is issued if a
condition does not depend on any of the variables captured by the
patterns.
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| | [1]
The reason why the blank spaces are mandatory with variables is that the XML
recommendation allows colons to occur in variables ("names" in XML terminology:
see section on XML Namespaces), so the blanks disambiguate
the variables. Actually only the blank on the right hand side is necessary:
CDuce accepts fun f (x1 :t1, ..., xn :tn):s =
e, as well (see also this paragraph on
let declarations in the tutorial). |
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