The underneath example works with
\def\myHeader{\iftuut chapter\else high\fi}

How do you make it work with\def\myHeader{\doifmodelse{a}{chapter}{high}} ?
( without resorting to \doifmodeelse{a}{\tuuttrue}{\tuutfalse} )

\showframe

\newif\iftuut
%\tuuttrue
\tuutfalse

\setuppagenumbering[location=footer]

\setupheadertexts[\doifoddpageelse{right page}{left page}]

\definetext[chapter][header][chapter page]

\def\myHeader{\doifmodelse{a}{chapter}{high}}
%\def\myHeader{\iftuut chapter\else high\fi}

\setuphead
 [chapter]
% [header=chapter]
% [header=high]
 [header=\myHeader]

\starttext
\dorecurse{2}{\chapter{Chapter #1}\dorecurse{20}{\input knuth\par}}
\stoptext

---

 ! Undefined control sequence.
\myHeader ->\doifmodelse
                         {a}{chapter}{high}
\doifvaluesomething ...inga {\csname #1\endcsname
                                                  }\ifx \!!stringa \empty
\e...
<argument> ...ething {\??ko \v!chapter \c!header }
                                                   {\setuplayouttext
[\v!hea...
\firstoftwoarguments #1#2->#1

<argument> ...BB {\v!chapter }\v!header \c!header
                                                  \dohandlepagebreakBB
{\v!c...
\firstofoneargument #1->#1

...
l.23 ...apter #1}\dorecurse{20}{\input knuth\par}}

---

Alan