Bob Glickstein > Blatte-0.9.4 > Blatte



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Blatte - text macro/markup/template language


  use Blatte;
  use Blatte::Builtins;

  my $perl = &Blatte::Parse('...some Blatte program ...');
  if (defined($perl)) {
    my $result = eval $perl;
    if (defined($result)) {
      &Blatte::traverse($result, \&callback);
    } elsif ($@) {
      ...handle execution error...
  } else {
    ...handle parsing error...


Blatte is a very powerful text markup and transformation language with a very simple syntax. A Blatte document can be translated into a Perl program that, when executed, produces a transformed version of the input document.

This module itself contains some utility functions for handling Blatte documents, described below in the FUNCTIONS section. However, writers of Blatte-based software will generally be more interested in other associated modules. See in particular Blatte::Compiler (for processing files full of Blatte code) and Blatte::Builtins (for a description of the Blatte language's intrinsic functions).

Most casual end users will probably be interested in Blatte's ability to serve as a higher-level language for writing web pages. This requires the additional CPAN package Blatte::HTML.

Most of the remainder of this document describes the syntax and semantics of the Blatte language.


Blatte has three metacharacters: \ { }. These are used to represent lists, variables, syntactic forms, function calls, string literals, comments, and a "forget-whitespace" operator explained below.

Everything else in a Blatte document is either whitespace or is divided into "words," also explained below.

To include a literal metacharacter, precede it with backslash: \\, \{, and \}.

All Blatte expressions correspond to equivalent Perl expressions. When a Blatte document is parsed, its Blatte expressions are converted to Perl, which may then be evaluated.

Here is a quick rundown of Blatte expression types.


This is a variable reference. The identifier following the \ must begin with a letter, and may be followed by letters, digits, or underscore.

This corresponds to the Perl scalar variable $var. All values in Blatte are Perl scalars.

{\define \VAR EXPR}

This defines a new variable \VAR to contain the value of the Blatte expression EXPR.

This corresponds to the Perl sequence

  use vars '$var';
  $var = EXPR;

(where EXPR has been transformed to its Perl equivalent).

{\set! \VAR EXPR}

This sets the existing variable \VAR to the value of the Blatte expression EXPR.

This corresponds to the Perl expression


This evaluates TEST. If the result is true, it evaluates and returns THEN; otherwise it evaluates the ELSEs and returns the value of the last one.

All Blatte values are true except for 0 and the empty string (as in Perl) and the empty Blatte list, {} (which corresponds to the Perl array reference []).

Perl equivalent:

  if (TEST) {
  } else {
{\and EXPR1 EXPR2 ...}

Evaluates each EXPR in turn, stopping if one yields a false value. Returns the value of the last EXPR it evaluates.

Perl equivalent:

  EXPR1 && EXPR2 && ...
{\or EXPR1 EXPR2 ...}

Evaluates each EXPR in turn, stopping if one yields a true value. Returns the true value if there is one, falsehood otherwise.

Perl equivalent:

  EXPR1 || EXPR2 || ...
{\cond {TEST1 THEN1a THEN1b ...} {TEST2 THEN2a THEN2b ...} ...}

Evaluates each TEST in turn. If one yields a true value, evaluates the corresponding THENs in sequence and returns the value of the last one.

Perl equivalent:

  if (TEST1) {
  } elsif (TEST2) {
  } ...
{\while TEST EXPR1 EXPR2 ...}

Evaluates TEST. If it's true, evaluates EXPR1 through EXPRn and starts again.

Perl correspondence:

  while (TEST) {
{\lambda {PARAM1 PARAM2 ...} EXPR1 EXPR2 ...}

Creates an anonymous subroutine. When the subroutine is invoked, its arguments will be assigned to the parameters PARAM1, PARAM2, etc., and the EXPRs evaluated in the resulting context. The value will be the value of the last EXPR.

Each PARAM is one of the following:


An ordinary variable reference. This creates a positional parameter.


This is a named parameter.


This is a rest parameter. There may be at most one of these.

See below for how to invoke Blatte subroutines, and how argument parsing proceeds.

Perl equivalent:

  sub {
    ...(argument parsing)...
{\define {\NAME PARAM1 PARAM2 ...} EXPR1 EXPR2 ...}

Defines a subroutine and assigns it to the variable \NAME. This is shorthand for

  {\define \NAME {\lambda {PARAM1 PARAM2 ...} EXPR1 EXPR2 ...}}
{\let {{\VAR1 VAL1} {\VAR2 VAL2} ...} EXPR1 EXPR2 ...}

Evaluates the VALs, then assigns them to the vars, then evaluates the EXPRs in the resulting context, returning the value of the last one.

Perl equivalence:

    my($VAR1, $VAR2, ...) = (VAL1, VAL2, ...);

{\let* {{\VAR1 VAL1} {\VAR2 VAL2} ...} EXPR1 EXPR2 ...}

Like \let, but each VAL is assigned to its \VAR as soon as it's computed, so later VALs can refer to earlier \VARs.

Perl equivalence:

    my $VAR1 = VAL1;
    my $VAR2 = VAL2;

{\letrec {{\VAR1 VAL1} {\VAR2 VAL2} ...} EXPR1 EXPR2 ...}

Like \let and \let*, but evaluates all the VALs in a context where the VARs have been declared but not yet assigned. This allows them to refer to one another by reference.

Perl equivalence:

    my($VAR1, $VAR2, ...);

    ($VAR1, $VAR2, ...) = (VAL1, VAL2, ...);


A sequence of one or more expressions enclosed in curly braces that isn't one of the expression types listed above is either a function call or a plain list.

If the value of EXPR1 is a Blatte subroutine, it is a function call, and the values of EXPR2 through EXPRn are passed as arguments. In this case it corresponds to the following Perl:

  &{EXPR1}({ ...named parameter hash... }, ...other arguments...)

See below for an explanation of Blatte subroutine argument parsing.

If EXPR1 isn't a Blatte subroutine, then

  {EXPR1 EXPR2 ... EXPRn}

is a plain list whose elements are the values of EXPR1 through EXPRn. Blatte lists correspond to Perl array references:

  [EXPR1, EXPR2, ..., EXPRn]
Delimited string

A Blatte string begins and ends with \" (\"sample\"). It may contain any characters, including whitespace. \ must be escaped like this: \\. Note, however, that " is not escaped.


Anything that contains no whitespace, isn't one of the above expression types, and escapes any instances of Blatte's three metacharacters, is a word.

In addition to the expression types above, there are two additional pieces of Blatte syntax:


This introduces a comment, which continues to the end of the line.


This is the "forget-whitespace" operator. It cancels any whitespace immediately preceding it. See the section on Whitespace handling below.

Argument parsing for function calls

As mentioned above, Blatte subroutines (a.k.a. functions) have three kinds of parameters: positional, named, and rest.

When calling a Blatte function, named parameters can be given values by writing


where \NAME is the parameter (which was given as \=NAME in the function definition).

All remaining arguments in the function call are assigned to positional parameters in the order in which they were declared. If there aren't enough positional parameters, then all the remaining arguments are collected in a Blatte list and assigned to the rest parameter.

When a Blatte function call is translated to Perl, the named parameter assignments are collected together in an anonymous HASH reference, which is passed as the first argument to the Perl subroutine corresponding to the Blatte function. (All Blatte functions are Perl subroutines that accept this anonymous HASH reference of named parameters as a first argument.) All remaining Blatte arguments are passed in sequence to the Perl subroutine.

Inside the Perl subroutine, arguments are unpacked as follows: named parameters are extracted from the HASH reference and assigned to correspondingly named Perl variables; Perl variables for the positional parameters get the next N arguments from @_; and the remainder of @_ is turned into an ARRAY reference (a Blatte list) and assigned to the Perl variable denoting the rest parameter.


  {\define {\fn \=n1 \=n2 \a \b \&r} stuff...}


  $fn = sub {
    my($_named, $a, $b) = splice(@_, 0, 3);
    my $n1 = $_named->{n1};
    my $n2 = $_named->{n2};
    my $r = \@_; stuff...

and the function call

  {\fn \n2=17 This is an example.}


  &$fn({n2 => 17}, 'This', 'is', 'an', 'example.')

which means $n1 will be undef, $n2 will be 17, $a will be 'This', $b will be 'is', and $r will be ['an', 'example'].

(Almost. This example ignores Blatte's whitespace handling, which is explained below.)

Whitespace handling

Before recognizing a Blatte expression, the Blatte parser skips over a (possibly empty) sequence of whitespace. This whitespace is preserved; then, when the parser is finished, the result is wrapped in a Blatte::Ws object, or a "whitespace wrapper," containing the preserved whitespace and the parsed expression. In this way, the whitespace preceding each expression is carried along with the expression itself. When Blatte expressions are parsed, evaluated, and rendered normally (see flatten() below), the output preserves the same whitespace found in the input.



Parses the first Blatte expression in INPUT using the default parser. The result is converted to a string of Perl code and returned.

(If you don't use the default parser, it's possible to change Blatte's syntax, to obtain the intermediate parse tree before conversion to Perl, and more. See Blatte::Parser(3).)

INPUT is either a string containing Blatte code, or a reference to such a string. If it's a reference, then after a successful parse, the matched expression will be removed from the beginning of the referenced string.

traverse(OBJ, CALLBACK, [WS])

Walks the data structure OBJ. If OBJ is an ARRAY reference, its elements are recursively traversed. If OBJ is a whitespace wrapper, its contents are recursively traversed. Otherwise, CALLBACK (a Perl CODE reference) is invoked on WS and OBJ. WS is a string of whitespace, possibly empty.

CALLBACK should return truth if it uses its whitespace argument and falsehood if it doesn't (meaning that traverse() should reuse the whitespace value in subsequent calls, if necessary). "Uses" usually means that the whitespace is copied to some sort of output.

This function supplies the logic that causes outer whitespace wrappers to take precedence over inner ones.

flatten(OBJ, [WS])

Renders OBJ as a string, with optional leading whitespace WS (which takes precedence over any outermost whitespace wrapper that might be present).

This function is written in terms of traverse().

wrapws(WS, OBJ)

Creates a new whitespace wrapper whose whitespace string is WS and whose nested object is OBJ.


Unwraps as many layers of whitespace wrapper from OBJ as necessary to reach a non-wrapper object, then returns that.


This returns the whitespace string, possibly empty, associated with OBJ. Note that

  wsof(wrapws(X, Y))

is always X, however

  unwrapws(wrapws(X, Y))

is Y only when Y is not itself a whitespace wrapper.


Returns the Blatte truth value of OBJ (after whitespace-unwrapping, see above). As in Perl, 0 and the empty string are false, but so is an empty Blatte list (that is, a Perl ARRAY ref with zero elements); all other values are true.


Quote STRING using Blatte syntax so that it can be read back as a Blatte string literal.


Blatte is a successor to an earlier language called Latte. The B is for "better" -- better because, whereas Latte's runtime facilities were fairly limited, Blatte's are effectively unlimited, since Blatte has the full power of Perl at its disposal. The implementation is also much faster and simpler than that of Latte. Latte users should beware that, despite being substantially similar, the Blatte language has significant differences from Latte.

The design of the Blatte language was strongly influenced by Scheme, and was guided by these principles:


Blatte is dedicated to the memory of Julie Epelboim, 1964-2001. Not that she ever would have used it. This was a woman who preferred to write raw Postscript code rather than use a page layout program. The tragedy of having lost her is dwarfed by the good fortune of having known her.


Bob Glickstein <>.

Visit the Blatte website, <>.


Copyright 2001 Bob Glickstein. All rights reserved.

Blatte is distributed under the terms of the GNU General Public License, version 2. See the file LICENSE that accompanies the Blatte distribution.


Blatte::Compiler(3), Blatte::Builtins(3), Blatte::Parser(3), Blatte::Ws(3).

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