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.\" ========================================================================
.\"
.IX Title "Catmandu::Fix::Bind 3pm"
.TH Catmandu::Fix::Bind 3pm "2019-01-29" "perl v5.28.1" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
Catmandu::Fix::Bind \- a wrapper for Catmandu::Fix\-es
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 3
\&  package Catmandu::Fix::Bind::demo;
\&  use Moo;
\&  with \*(AqCatmandu::Fix::Bind\*(Aq;
\&
\&  sub bind {
\&    my ($self,$data,$code,$name) = @_;
\&    warn "executing $name";
\&    $code\->($data);
\&  }
\&
\&  # in your fix script you can now write
\&  do
\&     demo()
\&
\&     fix1()
\&     fix2()
\&     fix3()
\&  end
\&
\&  # this will execute all the fixes as expected, and print to STDERR the following messages
\&
\&  executing fix1
\&  executing fix2
\&  executing fix3
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Bind is a package that wraps Catmandu::Fix\-es and other Catmandu::Bind\-s together. This gives
the programmer further control on the excution of fixes. With Catmandu::Fix::Bind you can simulate
the 'before', 'after' and 'around' modifiers as found in Moo or Dancer.
.PP
To wrap Fix functions, the Fix language introduces the 'do' statement:
.PP
.Vb 5
\&  do BIND
\&     FIX1
\&     FIX2
\&     FIX3
\&  end
.Ve
.PP
where \s-1BIND\s0 is a implementation of Catmandu::Fix::Bind and \s-1FIX1,...\s0,FIXn are Catmandu::Fix functions.
.PP
In the example above the \s-1BIND\s0 will wrap \s-1FIX1, FIX2\s0 and \s-1FIX3. BIND\s0 will first wrap the record data
using its 'unit' method and send the data sequentially to each \s-1FIX\s0 which can make inline changes
to the record data. In pseudo-code this will look like:
.PP
.Vb 5
\&  $bind_data = $bind\->unit($data);
\&  $bind_data = $bind\->bind($bind_data, $fix1);
\&  $bind_data = $bind\->bind($bind_data, $fix2);
\&  $bind_data = $bind\->bind($bind_data, $fix3);
\&  return $data;
\&
\& An alternative form exists, \*(Aqdoset\*(Aq which will overwrite the record data with results of the last
\& fix.
\&
\&  doset BIND
\&        FIX1
\&        FIX2
\&        FIX3
\&  end
.Ve
.PP
Will result in a pseudo code like:
.PP
.Vb 5
\&  $bind_data = $bind\->unit($data);
\&  $bind_data = $bind\->bind($bind_data, $fix1);
\&  $bind_data = $bind\->bind($bind_data, $fix2);
\&  $bind_data = $bind\->bind($bind_data, $fix3);
\&  return $bind_data;
.Ve
.PP
A Catmandu::Fix::Bind needs to implement two methods: 'unit' and 'bind'.
.SH "METHODS"
.IX Header "METHODS"
.SS "unit($data)"
.IX Subsection "unit($data)"
The unit method receives a Perl \f(CW$data\fR \s-1HASH\s0 and should return it, possibly converted to a new type.
The 'unit' method is called before all Fix methods are executed. A trivial, but verbose, implementation
of 'unit' is:
.PP
.Vb 5
\&  sub unit {
\&      my ($self,$data) = @_;
\&      my $wrapped_data = $data;
\&      return $wrapped_data;
\&  }
.Ve
.SS "bind($wrapped_data,$code)"
.IX Subsection "bind($wrapped_data,$code)"
The bind method is executed for every Catmandu::Fix method in the fix script. It receives the \f(CW$wrapped_data\fR
(wrapped by 'unit'), the fix method as anonymous subroutine and the name of the fix. It should return data
with the same type as returned by 'unit'.
A trivial, but verbose, implementaion of 'bind' is:
.PP
.Vb 7
\&  sub bind {
\&    my ($self,$wrapped_data,$code) = @_;
\&    my $data = $wrapped_data;
\&    $data = $code\->($data);
\&    # we don\*(Aqt need to wrap it again because the $data and $wrapped_data have the same type
\&    $data;
\&  }
.Ve
.SH "REQUIREMENTS"
.IX Header "REQUIREMENTS"
Bind modules are simplified implementations of Monads. They should answer the formal definition of Monads, codified
in 3 monadic laws:
.SS "left unit: unit acts as a neutral element of bind"
.IX Subsection "left unit: unit acts as a neutral element of bind"
.Vb 1
\&   my $monad = Catmandu::Fix::Bind\->demo();
\&
\&   # bind(unit(data), coderef) == unit(coderef(data))
\&   $monad\->bind( $monad\->unit({foo=>\*(Aqbar\*(Aq}) , $coderef) == $monad\->unit($coderef\->({foo=>\*(Aqbar\*(Aq}));
.Ve
.SS "right unit: unit act as a neutral element of bind"
.IX Subsection "right unit: unit act as a neutral element of bind"
.Vb 2
\&   # bind(unit(data), unit) == unit(data)
\&   $monad\->bind( $monad\->unit({foo=>\*(Aqbar\*(Aq}) , sub { $monad\->unit(shift) } ) == $monad\->unit({foo=>\*(Aqbar\*(Aq});
.Ve
.SS "associative: chaining bind blocks should have the same effect as nesting them"
.IX Subsection "associative: chaining bind blocks should have the same effect as nesting them"
.Vb 3
\&   # bind(bind(unit(data),f),g) == bind(unit(data), sub { return bind(unit(f(data)),g) } )
\&   my $f = sub { my $data = shift; $data\->{demo} = 1 ; $data };
\&   my $g = sub { my $data = shift; $data\->{demo} += 1 ; $data};
\&
\&   $monad\->bind( $monad\->bind( $monad\->unit({}) , f ) , g ) ==
\&     $monad\->bind( $monad\->unit({}) , sub { my $data = shift; $monad\->bind($monad\->unit($f\->($data)), $g ); $data; });
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
Catmandu::Fix::Bind::identity, Catmandu::Fix::Bind::benchmark