IfFileExists analogue in ConTeXt

There are various macros for checking if a file exists.

From file-ini.mkvi:

%D \macros
%D   {doifelsefile}
%D
%D The next alternative only looks if a file is present. No
%D loading is done. This one obeys the standard \TEX\ lookup.
%D
%D \starttyping
%D \doiffileelse {filename} {found} {not found}
%D \stoptyping

From file-res.mkvi:

%D \macros
%D   {readfile,ReadFile}
%D
%D One cannot be sure if a file exists. When no file can be
%D found, the \type{\input} primitive gives an error message
%D and switches to interactive mode. The macro \type{\readfile}
%D takes care of non||existing files. This macro has two faces.
%D
%D \starttyping
%D \ReadFile {filename}
%D \readfile {filename} {before loading} {not found}
%D \stoptyping
%D
%D Many \TEX\ implementations have laid out some strategy for
%D locating files. This can lead to unexpected results,
%D especially when one loads files that are not found in the
%D current directory. Let's give an example of this. In
%D \CONTEXT\ illustrations can be defined in an external file.
%D The resizing macro first looks if an illustration is defined
%D in the local definitions file. When no such file is found,
%D it searches for a global file and when this file is not
%D found either, the illustration itself is scanned for
%D dimensions. One can imagine what happens if an adapted,
%D localy stored illustration, is scaled according to
%D dimensions stored somewhere else.
%D
%D When some \TEX\ implementation starts looking for a file, it
%D normally first looks in the current directory. When no file
%D is found, \TEX\ starts searching on the path where format
%D and|/|or style files are stored. Depending on the implementation
%D this can considerably slow down processing speed.
%D
%D In \CONTEXT, we support a project||wise ordening of files.
%D In such an approach it seems feasible to store common files
%D in a lower directory. When for instance searching for a
%D general layout file, we therefore have to backtrack.
%D
%D These three considerations have lead to a more advanced
%D approach for loading files.
%D
%D We first present an earlier implementation of
%D \type{\readfile}. This command backtracks parent
%D directories, upto a predefined level. Users can change this
%D level (on the commandline using a directive); we default to~3.
%D
%D We use \type{\normalinput} instead of \type{\input}
%D because we want to be able to redefine the original
%D \type{\input} when needed, for instance when loading third
%D party libraries.

....

%D \macros
%D   {readjobfile,readlocfile,readsysfile,
%D    readfixfile,readsetfile}
%D
%D This implementation honnors the third situation, but we
%D still can get unwanted files loaded and/or can get involved
%D in extensive searching.
%D
%D Due to different needs, we decided to offer four alternative
%D loading commands. With \type{\readjobfile} we load a local
%D file and do no backtracking, while \type{\readlocfile}
%D backtracks~\number\maxreadlevel\ directories, including the current
%D one.
%D
%D System files can be anywhere and therefore
%D \type{\readsysfile} is not bound to the current directory
%D and obeys the \TEX\ implementation.
%D
%D Of the last two, \type{\readfixfile} searches on the
%D directory specified and backtracks too, while
%D \type{\readsetfile} does only search on the specified path.
%D
%D The most liberal is \type {\readfile}.

....

%D So now we've got ourselves five file loading commands:
%D
%D \starttyping
%D \readfile                {filename} {before loading} {not found}
%D
%D \readjobfile             {filename} {before loading} {not found}
%D \readlocfile             {filename} {before loading} {not found}
%D \readfixfile             {filename} {before loading} {not found}
%D \readsysfile {directory} {filename} {before loading} {not found}
%D \stoptyping

...

%D \macros
%D   {doiflocfileelse,locfilename}
%D
%D \starttyping
%D \doiflocfileelse {filename} {before loading} {not found}
%D \stoptyping

You can also check for file existence on the Lua end. If you are inside a Lua snippet when you want to test, this might be more convenient.

\startluacode
local fname = "foo.bar"
local f = io.open(fname)
if f == nil then
   tex.error("Could not open file " .. fname)
end
\stopluacode

If you want to search the whole TeX path for the file named fname use

local f = io.open(kpse.find_file(fname))