path: root/macros/luatex/optex/fonts-select.opm

%% This is part of OpTeX project, see http://petr.olsak.net/optex

\_codedecl \fontfam {Fonts selection system <2020-03-18>} % preloaded in format

\_def\_initunifonts {%
   \_ea\_newcount \_csname e@alloc@attribute@count\_endcsname
   \_global \_csname e@alloc@attribute@count\_endcsname=-1   
   \_directlua{%
      require("ltluatex")
      require('luaotfload-main') local _void = luaotfload.main ()
   }%
   \_gdef\_rfskipatX ##1" ##2\_relax{"##1"}% 
   \_global\_let \_doresizefont=\_doresizeunifont
   \_gdef\_tryloadtt {\_fontdef\_tentt{\def\_fontnamegen{[lmmono10-regular]}\_rm}}% 
   \_global\_let \_initunifonts=\_relax % we need not to do this work twice
   \_global\_let \initunifonts=\_relax
}
\_gdef\_doresizeunifont #1{\_logfont{#1}%
   \_ifx\_fontnamegen\_undefined \_doresizetfmfont#1\_else
       \_font#1={\_fontnamegen} \_sizespec \_relax \_setwsp#1\_relax
   \_fi
}
\_def\_fontdecl [#1]#2#3#4#5#6#7{%
   \_initunifonts \_uniaccents
   \_ifx #2\_undefined 
      \_isfont{#7}\_iffalse
         \_opwarning{Family [#1] skipped, font "#7" not found}\_ea\_ea\_ea\_endinput \_else
         \_def\_textfamily{#1}%
         \_edef\_currfamily {\_csstring #2}%
         \_def\_mainfamcommand{#2}\_def\_mathfaminfo{#6}%
         \_wterm {FONT: [#1] -- \string#2 \_detokenize{(#3)^^J mods:{#4} vars:{#5} math:{#6}}}%
      \_fi 
   \_else \_ea #2\_ea\_endinput \_fi
}
\_def\_regoptsizes #1 #2?#3 #4*{\_sdef{_optsizes:#1}{#2?#3 #4* }}
\_def\_optfontalias #1#2{\_slet{_optsizes:#1}{_optsizes:#2}}
\_def\_optname #1{\_ifcsname _optsizes:#1\_endcsname
   \_ea\_ea\_ea \_optnameA \_csname _optsizes:#1\_ea\_endcsname
   \_else \_failedoptname{#1}\_fi
}
\_def\_failedoptname #1{optname-fails:(#1)}
\_def\_optnameA #1?#2 #3 <#4 {\_ifx*#4#1#3#2\_else
   \_ifdim\_optsize<#4pt #1#3#2\_optnameC
   \_else \_afterfifi \_optnameA #1?#2 \_fi\_fi
}
\_def\_optnameC #1* {\_fi\_fi}
\_def\_afterfifi #1\_fi\_fi{\_fi\_fi #1}

\_newifi \_ifexistfam
\_def\_isfont#1#2{%
   \_begingroup
       \_suppressfontnotfounderror=1
       \_font\_testfont={#1}\_relax
       \_ifx\_testfont\_nullfont \_def\_tmp{\_existfamfalse \_unless}
       \else \_def\_tmp{\_existfamtrue}\_fi
   \_ea \_endgroup \_tmp #2%
}

\_def \_defaultfontfeatures {+tlig;}
\_def \_setff #1{%
   \_ifx^#1^\_let \_fontfeatures=\_defaultfontfeatures
   \_else \_edef\_fontfeatures{\_fontfeatures #1;}\_fi
   \_reloading
}
\_setff {} % default font features: +tlig;
\_def\_removefeature #1{%
    \_isinlist\_fontfeatures{#1}\_iftrue
        \_def\_tmp ##1#1##2;##3\_relax{\_def\_fontfeatures{##1##3}}%
        \_ea \_tmp \_fontfeatures \_relax 
    \_fi
}
\_def\_fvars #1 #2 #3 #4 {%
   \_sdef{_fvar:rm}{#1}%
   \_sdef{_fvar:bf}{#2}%
   \_ifx.#2\_slet{_fvar:bf}{_fvar:rm}\_fi
   \_sdef{_fvar:it}{#3}%
   \_ifx.#3\_slet{_fvar:it}{_fvar:rm}\_fi
   \_sdef{_fvar:bi}{#4}%
   \_ifx.#4\_slet{_fvar:bi}{_fvar:it}\_fi
}
\_def\_currV{\_cs{_fvar:\_whatresize}}
\_def\_V{ }
\_def \_fsetV #1 {\_fsetVa #1,=,}
\_def \_fsetVa #1=#2,{\_isempty{#1}\_iffalse
   \_ifx,#1\_else\_sdef{_#1V}{#2}\_ea\_ea\_ea\_fsetVa\_fi\_fi
}
\_def \_onlyif #1=#2:#3{%
    \_edef\_act{\_noexpand\_isinlist{,#2,}{,\_cs{_#1V},}}\_act
    \_iftrue #3\_fi
}
\_def \_moddef #1#2{\_edef\_tmp{\_csstring#1}\_remfirstunderscore\_tmp
   \_sdef{_f:\_currfamily:\_tmp}{#2\_reloading}%
   \_protected \_edef #1{\noexpand\_famdepend\noexpand#1{_f:\noexpand\_currfamily:\_tmp}}%
   \_ea \_ifx \_csname\_tmp\_endcsname #1\_else
      \_ea \_public \_csname\_tmp\_endcsname ;\_fi
}
\_protected \_def\_resetmod {\_cs{_f:\_currfamily:resetmod}} % private variant of \resetmod
\_def\_currfamily{} % default current family is empty

\_def\_famdepend#1#2{\_ifcsname#2\_endcsname \_csname#2\_ea\_endcsname \_else
   \_opwarning{\string#1 is undeclared in current family "\_currfamily", ignored}\_fi
}
\_def\_famvardef#1{\_edef\_tmp{\_csstring#1}\_remfirstunderscore\_tmp
   \_ea\_famvardefA \_ea#1\_csname _ten\_tmp\_ea\_endcsname
      \_csname _tryload:\_tmp\_endcsname
}
\_def\_famvardefA #1#2#3#4{% #1=\_XX #2=\_tenXX #3=\_tryloadXX #4=data
   \_isinlist{.\_rm\_bf\_it\_bi\currvar\_currvar}#1\iftrue
      \_opwarning{\string\famvardef: 
          You cannot re-declare private standard variant selector \string#1}%
   \_else
      \_protected\_edef #1{\noexpand\_famdepend\noexpand#1{_f:\noexpand\_currfamily:\_tmp}}%
      \_sdef{_f:\_currfamily:\_tmp}{#3#2}%
      \_def#3{\_fontdef#2{#4}}%
      \_ifx#1\tt \addto#1{\_fam\_ttfam}\fi
      \_sdef{_currvar:\csstring#2}{#1}%
   \_fi
}
\_def\_initfontfamily{%
   \_mainfamcommand \_reloading \_rm   
}
\def\_fontfam[#1]{%
   \_lowercase{\_edef\_famname{\_ea\_removespaces #1 {} }}%
   \_ifcsname _famf:\_famname\_endcsname \_edef\_famfile{\_cs{_famf:\_famname}}%
   \_else \_edef\_famfile{f-\_famname}\_fi
   \_ifx \_famfile\_empty \_listfamnames \_fi
   \_isfile {f-\_famname.opm}\_iftrue
      \_opinput {f-\_famname.opm}
   \_else \_isfile {fams-local.opm}\_iftrue \_opinput {fams-local.opm} \_fi
      \_ifcsname _famf:\_famname\_endcsname \_edef\_famfile{\_cs{_famf:\_famname}}%
      \_else \_edef\_famfile{f-\_famname}\_fi
      \_isfile {\_famfile.opm}\_iftrue \_opinput {\_famfile.opm}
      \_else \_opwarning{Family [#1] undeclared, "\_famfile.opm" not found}%
   \_fi \_fi \_relax
}
\_sdef{_famf:?}{} \_sdef{_famf:{} }{}
\_def\_listfamnames #1\_fi\_relax{\_fi
   \_wterm{===== List of font families ======}
   \_begingroup
       \_let\_famtext=\_wterm
       \_def\_faminfo [##1]##2##3##4{%
           \_wterm{ \_space\_noexpand\fontfam [##1] -- ##2}%
       \_let\_famalias=\_famaliasA}%
       \_opinput {fams-ini.opm}
       \_isfile {fams-local.opm}\_iftrue \_opinput {fams-local.opm} \_fi
       \_message{^^J}%
   \_endgroup
}
\_def\_famaliasA{\_message{ \_space\_space\_space\_space -- alias:}
   \_def\_famalias[##1]{\_message{[##1]}}\_famalias}

\_def\_famtext #1{}
\_def\_faminfo [#1]#2#3#4{%
   \_lowercase{\_edef\_tmp{\_ea\_removespaces #1 {} }}%
   \_sdef{_famf:\_tmp}{#3}%
   \_def\_famfile{#3}%
}
\_def\_famalias [#1]{%
   \_lowercase{\_edef\_famname{\_ea\_removespaces #1 {} }}%
   \_sdef{_famf:\_famname\_ea}\_ea{\_famfile}%
}   
\_input fams-ini.opm
\_let\_famfile=\_undefined
\_newtoks \_catalogsample
\_newtoks \_catalogmathsample 
\_newtoks \_catalogonly 
\_newtoks \_catalogexclude
\_catalogsample={ABCDabcd Qsty fi fl áéíóúüů řžč ÁÉÍÓÚ ŘŽČ 0123456789}

\_def\_savedfontcolor{}
\_def\_savedletterspace{}
\_def\_savedwsp{}

\_def \_setfontcolor #1{\_removefeature{color=}%
    \_edef\_tmp{\_calculatefontcolor{#1}}%
    \_ifx\_tmp\_empty \_else \_edef\_fontfeatures{\_fontfeatures color=\_tmp;}\_fi
    \_reloading
}
\_def \_setletterspace #1{\_removefeature{letterspace=}%
    \_if^#1^\_else \_edef\_fontfeatures{\_fontfeatures letterspace=#1;}\_fi
    \_reloading
}
\_def \_setwordspace #1{%
   \_if^#1^\_def\_setwsp##1{}\_removefeature{+Ws}% 
   \_else \def\_setwsp{\_setwspA{#1}}\_setff{+Ws}\_fi
   \_reloading
}
\_def\_setwsp #1{}
\_def\_setwspA #1#2{\_fontdimen2#2=#1\_fontdimen2#2%
   \_fontdimen3#2=#1\_fontdimen3#2\_fontdimen4#2=#1\_fontdimen4#2}

\_def\_calculatefontcolor#1{\_trycs{_fc:#1}{#1}} % you can define more smart macro ...
\_sdef{_fc:red}{FF0000FF}    \_sdef{_fc:green}{00FF00FF} \_sdef{_fc:blue}{0000FFFF}
\_sdef{_fc:yellow}{FFFF00FF} \_sdef{_fc:cyan}{00FFFFFF}  \_sdef{_fc:magenta}{FF00FFFF}
\_sdef{_fc:white}{FFFFFFFF}  \_sdef{_fc:grey}{00000080}  \_sdef{_fc:lgrey}{00000025}
\_sdef{_fc:black}{} % ... you can declare more colors...

\_public
   \fontfam \initunifonts \isfont 
   \setff \setfontcolor \setletterspace \setwordspace \famvardef \moddef 
   \catalogonly \catalogexclude \catalogsample \catalogmathsample ;

\_endcode %---------------------------------------------------


\sec Usage of font selection system
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Try to write `\fontfam[?]`. All font families registered in \OpTeX/
font selection system are listed on the terminal and in the log file.
You can choose one of them and use it, for example `\fontfam[Termes]` or
`\fontfam[LM Fonts]`. Then you can use four basic variant selectors `\rm` or
or `\bf` or `\it` or `\bi` and desired fonts are printed.

You can write `\fontfam[Catalog]` and all fonts registered in \OpTeX/ are
printed with their templates.

The fonts registered in \OpTeX/ have their macros in "font files", each family
in one font file with the name `f-famname.opm`. All families are collected
in `fams-ini.opm`. You can read the documentation in the file `fams-ini.opm`
and in the files `f-heros.opm`, `f-lmfonts.opm` for more information how
to install and how to register your own font family. Note that all families
registered in such files are Unicode fonts. Non-Unicode fonts are not
preferred in \OpTeX/ but you can use them using classical `\font` primitive,
of course.

There are two equal possibilities how to load a font family: `\fontfam[Family]`
or `\input f-fontfamily.opm`.

There are "main family command", "font modifiers" and "variant selectors" in
each family. Each family provides four default variant selectors mentioned above
(may be, in few cases a missing variants are substituded)
A few families provides more than these four variant selectors.

Mostly all families provides "font modifiers" like `\caps`, `\light`,
`\cond`. Supported font modifiers are listed in terminal or in the
catalogue. A font modifier does nothing with current font, but the next
variant selectors select a modified font. You can combine more than one
font modifiers followed by a variant selector. The font is reloaded at the
moment when variant selector is processed. The following example
supposes that we are working with a font family where `\caps` and `\cond`
font modifiers are provided.

\begtt
\rm normal font, \bf bold, \it italic.
\cond\currvar now condensed italics is active,
\rm condensed normal, \bf condensed bold.
\caps\rm condensed caps and small caps variant is used in regular variant
\bi condensed caps and small caps variant is used in bold-italic variant.
\resetmod\rm using the `\resetmod` font modifier we can return to 
the normal state when no font modifier is active.
\endtt

Of course, all font settings respect groups. The font modifiers keeps their
context until the group is closed or until another font modifier (which
negates previous setting) is used. In our example these two selectors are
independent, so there are four variants: normal, condensed, caps and
caps-condensed, all these variants are present in four standard 
variants: regular, bold, italics, bold-italics.

There is one special command `\currvar` which acts as variant selector. 
It keeps the current variant and the font of such variant is 
reloaded with respect to previously given font modifiers. 

You can use the `\setfontsize{<sizespec>}` command in the same sense as
other font modifiers. It means that the command itself does nothing with the
fonts but following variant selectors reload fonts to desired size. For
example:

\begtt
\rm standard size \setfontsize{at14pt}\rm here is 14pt size \it italic is 
in 14pt size too \bf bold too.
\endtt

Note that much more comfortable is to resize fonts using OPmac-like command
`\typosize`, `\typoscale`. These commands prepare the right sizes for math
fonts too and re-calculates many internal parameters like `\baselineskip`. See
the section \ref[typosize] for more information.


\secc More than one font families loaded
%---------------------------------------

You can load more font families. Then you must to know the context (what
family is currently in use) and you can switch between families using main
family comand. For example, when Heros and Termes are loaded then:

\begtt
\rm here is Termes because it is loaded last, \it italics in Termes but
\Heros \rm here is Heros font \it italics is in Heros too.
\endtt

To select a font you can use (in this order):

\begtt
\<main-family-command> \<font-modifiers> \<variant-selector-or-currvar>
\endtt

You can use zero or more font modifiers, the `\setfontsize` command is a
font modifier too. Finally, the variant selector or `\currvar` loads 
the desired font. 

The font modifiers (with an exception `\setfontsize`) are declared localy in
their font family as "family dependent". If you use it in the context of
another font family where it is not declared then warning is printed on the
terminal and nothing else happens. It is possible (and it is typical) that a
modifier with the same name is declared in more font families. Then such
modifiers are working in context of current used family. For example
`\LMfonts\caps\rm` or `\Termes\caps\rm` both loads small caps in specified
font family using different internal definitions.

Maybe, one needs a context switching between `\rm`<-->`\bf` and
`\it`<-->`\bi`. This is done by `\em` and `\boldify` macros, see 
section \ref[fonts].


\secc Math Fonts
%---------------

Most font families are connected with a preferred unicode-math font. This
unicode-math is activated when the font family is loaded. If you don't prefer
this and you are satisfied with 8bit math CM+AMS fonts preloaded in the
\OpTeX/ format then you can use command `\noloadmath` before you load a first
font family. 

If you want to use your specially selected UnicodeMath font then use
`\loadmath{[<font_file>]}` or `\loadmath{<font_name>}` before first
`\fontfam` is used.


\secc Defining a font commands
%-----------------------------

You can declare a "font switch" like `\font` primitive does it, 
but you need not to know nothing about names of font files:

\begtt
\fontdef \newswitch {<font-modifiers> <variant-selector>}
for example
\fontdef \titlefont {\setfontsize{at14pt}\bf}
\endtt

This example declares \title as a font switch: it selects the bold font at
14pt and other not mentioned parameters (font family and font modifiers) are 
taken from outside context when `\fontdef` is in process. 
The result is a "definitive" font switch (internaly declared by `\font` primitive) 
which does not respect any other current context when it is used. It means:

\begtt
\sizespec{at8pt}\rm small roman font \titlefont bold big font \it italics 
small font.
\endtt

Such font switches sould be used in `\output` routine (headers, footers) for
example. But it should be less usable in common text. For example when
`\titlefont` defined above is used for titles, you cannot change a font
variant (to `\it`, for example). So, better idea to declare a "title font"
is to use normal `\def` instead `\fontdef`

\begtt
\def\titlefont{\setfontsize{at14pt}\bf \let\it\bi} % Let it be (John Lennon)
...
{\titlefont here we have bold 14pt font and {\it here} was bold 14pt italics}
\endtt

There are four standard variant selectors, but you can declare new variant
selector by `\famvardef` command:

\begtt
\famvardef \selector {<font-modifiers> <variant-selector>}
\endtt

If such `\selector` is used, then it takes the current context of font family and
font modifiers, opens a group, locally adds the defined font modifiers, 
applies variant selector (loads the font now), closes the group but keeps
the selected font and use it. Moreover, such `\selector` is declared as
family-dependent in the same sense as font modifiers.


\secc Example of modifying font features
%---------------------------------------

Each OTF font provides "font features". You can list these font features
by `otfinfo -f font.otf`. For example LinLibertine fonts provide `frac` font
feature. If it is active then fractions like 1/2 are printed in a special
form.

The macro `\setff{<feature>}` acts like family independent font modifier and
prepares a new <feature>. You must use a variant selector in order to
reinitialize the font with the new font feature. For example
`\setff{+frac}\rm` or `\setff{+frac}\currvar`. You can declare a new variant
selector too:

\begtt
   \fontfam[LinLibertine]
   \famvardef \fraclig {\setff{+frac}\currvar}
   Compare 1/2 or 1/10 \fraclig to 1/2 or 1/10.
\endtt

If the used font does not supports given font feature then font is realoaded
without warning nor error. Silently: font feature is not activated.

The `onum` font feature (old style digits) is connected to `\caps` macro for
Caps+SmallCaps variant in \OpTeX/ font-definition-files. So you need not to
create a new modifier, just use `{\caps\currvar 012345}`.


\secc Special font modifiers
%---------------------------

Despite the font modifiers declared in the font-file (and dependent on 
the font family), we have following font modifiers:

\begtt
\setfontsize{<sizespec>}   % sets the font size
\setff{<font_feature>}     % adds the font feature
\setfontcolor{<color>}     % sets font color
\setletterspace{<number>}  % sets letter spacing
\setwordspace{<scaling>}   % modifies word spacing
\endtt

The `\setfontsize` command is described in section \ref[fontsize].
The `\setff` command was described in previous subsection.

`\setfontcolor{<color>}` specifies the color and the opacity of the text.
The <color> parameter should be in hexadecimal format of four bytes
`<red><green><blue><opacity>`, for example `FF0080FF` means full red, zero
green, half blue and full opacity. You can use names `red`, `green`, `blue`,
`yellow`, `cyan`, `magenta`, `white`, `grey`, `lgrey` instead of the hexadecimal
specification. The emtpy parameter <color> means default black color.

That colors of fonts are implemented using \LuaTeX/ internal font feature. This
is different approach than using colors in section \ref[colors].

`\setletterspace{<number>}` specifies letter spacing of the font. The
<number> is decimal number without unit. The unit is supposed as 1/100 of
the font size. I.e. `2.5` means 0.25 pt when the font is at 10 pt size. The
empty parameter <num> means no letter spacing which is default.

`\setwordspace{<scaling>}` scale the default interword space (defined in the
font) and its stretching and shrinking parameters by given <scaling>
factor. For example `\setwordspace{2.5}` multiplies interword space by 2.5.

If you need another font transformations, you can use `\setff` 
with following font features provided by Lua\TeX/:

\begtt
   \setff{embolden=1.5}\rm % font is bolder because outline has nonzero width
   \setff{slant=0.2}\rm    % font is slanted by a linear transformation
   \setff{extend=1.2}\rm   % font is extended by a linear transformation.
\endtt

Use font transformations mentioned above and `\setletterspace`,
`\setwordspace` with care. The best setting of these values is default
setting in every font, of course. If you really needs to set a different
letter spacing then it is strongly reccomended to add `\setff{-liga}` in
order to disable ligatures. And setting a positive letter spacing probably
needs to scale interword spacing too.

All mentioned font modifiers (with the exception of `\setfontsize`) work only
with Unicode fonts loaded by `\fontfam`.


\secc Example of usage more vaiants
%----------------------------------

Suppose we have load Heros family where \cond font modifier is declared.
Then you can define:

\begtt
  \famvardef \rmcond {\cond\rm}
  \famvardef \bfcond {\cond\bf}
  \famvardef \itcond {\cond\it}
  \famvardef \bicond {\cond\bi}
\endtt

Now, you can select between eight variants. But the context between similar
variants like `\bf`--`\bfcond` is lost. Maybe you will find more usefull to
create only a two cond--nocond simple macros which respect the current variant.
And use only four standard variant selectors:

\begtt
  \def\useCond {\cond\currvar}  \def\useNormal {\nocond\currvar}
  \Heros\rm 
  here is rm-normal \bf here is bold-normal \useCond here is bold-condensed
  \it here is italics-condensed \useNormal here is italics-normal.
\endtt

Another example of decalaration of `\mr` (medium-regular) and `\mi`
(medium-italics) variant selectors is in the file f-lido.opm.


\secc Example of usage more than one family
%------------------------------------------

Supose we have load Termes and Heros families. Then you can define

\begtt
   \def\useTermes {\Termes\currvar} % switch to Termes with respect current variant
   \def\useHeros  {\Heros\currvar}  % switch to Heros with respect curent variant
\endtt

Or you can declare font selectors:

\begtt
   \fontdef \titlefont {\setfontsize{at14pt}\Heros\bf}
   \times\rm ..... normal text in Times
   \titlefont .... Titles in Heros.
\endtt

If you are mixing fonts from various font families then the ex height can
be different and optical unfit. You can compensate exheight of such fonts 
by `mag` keyword in the `\setfontsize` macro. For example by:

\begtt
   \Termes
   \famvardef \sansfont {\setfontsize{mag.9}\Heros\currvar}
   \rm here is text in Termes \sansfont and text in Heros with compatible ex-height.
\endtt


\secc \code{\\fontfam} processing in detail
%------------------------------------------

The `\fontfam` macro lists all declared families if parameter is empty or
`?`. If not, then `\fontfam` transforms the given parameter to lowercase and
without spaces. If exists the file `f-<parameter>.opm` then it is read. If
does not exists such file then `\fontfam` internally lists the families
declared by `\_faminfo` and `\_famalias`, firstly from `fams-ini.opm` file
and secondly from `fams-local.opm` (if such file exists). The last
declaration from this internal list (which mathes the given <parameter>)
wins. If the parameter does not match then font-macro-file is not found and 
the warning is printed and no new family is loaded.

When the font-macro-file is found, then `\fontfam` tries an existence of fonts in the
system. If they does not exist, a warning on the terminal is printed and no
family is loaded.

Note, that `fams-local.opm` has higher precedence that internal
`fams-ini.file`. You can declare families with the same name as internal
names and your declaration wins.


\secc Others

You can use `\isfont {<fontname>}\iftrue` or `\isfont {<fontname>}\iffalse`
to test if the specified font is present in your system.

The macro `\initunifonts` should be used before usage of first `\font` primitive
when you need to lodad Unicode fonts directly wihout support of the `\fontfam` or
`\loadmath` \"high level" macros\fnote
{The `\initunifonts` macro initializes an extension features of the
\code{\\font} primitive using Lua code. Without this only 8\,bit fonts can be
loaded.}.

The `\boldify` macro can be used for titles. It sets `\it` to `\bi` and
`\rm` to `\bf`. It means that user can write `{\it something}` in the title
and the result is the same as `{\bi something}`. Math formulae are in bold 
variant too. The `\boldify` macro has local validity like others font 
modifiers. Example:

\begtt
{\boldify\typosize[14/16] Title in {\it bold}\par}
\endtt



\secc Implementation notes
%-------------------------

TODO ...

The family-dependend macros are declared by \moddef and \famvardef. The
following trick is used:

\begtt
    \protected\def\macro{\_famdepend\macro{_f:\_currfamily:macro}}%
    \sdef{_f:currfamily:macro}{... definition body ...}%
\endtt

The `\_famdepend\macro{csname}` runs `\csname` if it is defined else it writes
a warning. 

If you want to declare macros wit the same feature but wit a parameter, for
example, you have to do:

\begtt
   \protected\def\macro{\famdepend\macro{_f:\_currfamily:macro}}%
   \sdef{_f:currfamily:macro}#1{... definition body ...}%
\endtt

Of course, you must to do some `\edef...\noexpand` tricks but this is an
implementation detail.