newif conditional in align environment to optionally hide sub-steps in derivation

Question

I created an if true/false newififdetail with the hope of using this inside a begin{align}...end{align} environment.  The goal is, I'm converting my old hand-written derivations to tex, and wanted to be able to flip a macro in order to show or hide all the intermediate steps of the derivation (example, hide details in main text and put all the details in an appendix by changing the true/false declaration either in different files, or when putting the details in an appendix etc).
My attempt so far fails (below).  Does anyone have a working solution to such a set up?  The use of align and/or eqnarray are essential for multi-line equations.
When I try to get this to work inside an align, I can either get the false or true version to work (by wrapping the conditional with {} in the former case, but not both simultaneously.  It seems align is doing some things that conflict with this simplistic newif as it causes compilation errors.  I have two minimal-working-examples (MWEs) that demonstrate these two cases.
% This MWE compiles 
% true:  compiles
% false: fails
documentclass{article}
usepackage{amsmath}

% new if true/false
newififdetail
% set it to true, this works
%letifdetailiftrue

% set it to false, this fails
letifdetailiffalse

begin{document}

begin{align}
a &=b 
ifdetail
nonumber&=
textrm{intermediate step}
fi
nonumber&=
textrm{final answer}
end{align}

end{document}

% This MWE compiles  - notice the extra {} surrounding the {ifdetail...fi}
% true:  fails
% false: compiles
documentclass{article}
usepackage{amsmath}

% new if true/false
newififdetail
% set it to true, this works
letifdetailiftrue

% set it to false, this fails
%letifdetailiffalse

begin{document}

begin{align}
a &=b
{ifdetail
nonumber&=
textrm{intermediate step}
fi}
nonumber&=
textrm{final answer}
end{align}

end{document}

Steven B. Segletes · Accepted Answer

I have to hide the conditional code from align by keeping it in a group (argument), until the last minute.
So in the align block, I call on testifdetail with the conditional code as an argument (key: in braces).  Then testifdetail performs the if test, but still shields the argument by invoking a call to testifdetailaux only ifdetail is true, and then by way of argument (again, in braces).
Finally, clear of the align search logic, testifdetailaux is able to spit out the result directly into the align block.
% This MWE compiles 
% true:  compiles
% false: fails
documentclass{article}
usepackage{amsmath}

% new if true/false
newififdetail
% set it to true, this works
detailtrue

% set it to false, this fails
%detailfalse

newcommandtestifdetail[1]{ifdetailtestifdetailaux{#1}fi}
newcommandtestifdetailaux[1]{#1}

begin{document}

begin{align}
a &=b 
testifdetail
{nonumber&=
textrm{intermediate step}
}
nonumber&=
textrm{final answer}
end{align}

detailfalse

begin{align}
a &=b 
testifdetail
{nonumber&=
textrm{intermediate step}
}
nonumber&=
textrm{final answer}
end{align}

end{document}

Willie Wong · Answer

This has something to do with where you put the line breaks. Additionally, this is not a align specific problem: if you do a similar version of your code using straight-up tabular, you get exactly the same error message. But if you move the line breaks the way I do, you are okay.
The following code works for both detailtrue and detailfalse; all I did is move the nonumber around.
documentclass{article}
usepackage{amsmath}

% new if true/false
newififdetail
detailtrue

begin{document}

begin{align}
a &=b nonumber 
ifdetail
&=
textrm{intermediate step} nonumber
fi
&=
textrm{final answer}
end{align}
end{document}

For the benefit of smarter wizards: the same problem as reported by the OP shows up in
documentclass{article}

newififdetails
detailstrue

begin{document}
begin{tabular}{cc}
        a & 1 
        ifdetails
         b & 2 
        c & 3 
        d & 4 
        fi
         e & 5
end{tabular}
end{document}

but all works well with
documentclass{article}

newififdetails
detailsfalse

begin{document}
begin{tabular}{cc}
        a & 1 
        ifdetails
        b & 2 
        c & 3 
        d & 4 
        fi
        e & 5
end{tabular}
end{document}

wipet · Answer

When a conditional (ifdetails in your example) starts inside the halign item after first non-expadable token, then following tokens are put to the input queue until the & or cr (in LaTeX it is ) occurs. Then the input queue follows by the second part of column declaration data finalized by endtemplate which is un-skippable by if...fi couple, so error arises. This is your example which can be simplify by this example:
halign{x#ycr
   aiffalse b cr
   cfi cr
}

The iffalse here skips b but then cr is replaced by the second part of column data (i.e. y) followed by endtemplate, so we have error.
The working examples shown by Steven Segletes is based on the fact, that his conditionals start at the beginning of the template (spaces are ignored and tokens are expanded at this state, because TeX tries to find the omit primitive).
The & and cr are not replaced by the second part of column data in this state. So, they are skipable by the conditional. The example can be simplify by:
halign{x#ycr
  iffalse b cr
  fi c cr
}

Universal principle, how to skip a part of table data, is based on the fact, that TeX allow to read cr or & into a macro parameter (without replacing them by second part of the column data), if it is processed at inside-braces level. This feature is implemented because TeX allows to do matrix{} in matrix{}, for example. It means that this works:
defignore#1{}
halign{x#ycr
  aignore{ b cr
  c} cr
}

Your example should be solved by following code:
defignore#1{} defuse#1{#1}
letprocess=ignore   %
%letprocess=use     % select, what you want

and use it in your aligning macro (which is nothing other than halign primitive):
begin{align}
a &=b 
process{%
   nonumber&=
   textrm{intermediate step}
}
nonumber&=
textrm{final answer}
end{align}

newif conditional in align environment to optionally hide sub-steps in derivation

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