Ggtkuk

I'm trying to write myself a package, similar to the exercise package, so I can prepare problem sets for my students. I wish to delay the output of problems and/or solutions until a suitable place in the document has been reached. I have something that sort of works but it's kind of clunky. For example, this works pretty well:

usepackageenviron
NewEnvirontestbglobalexpandafterletcsname barendcsnameBODY

I use csname because the names are dynamically generated. This doesn't work so well if the command is anything other than BODY. For example, if I do letbarBODY, latex has an aneurysm (it binds bar to just {).

There is an alternate way of doing this, which is:

NewEnvirontestcglobalexpandafteredefcsname fooendcsnameBODY

This sort of works. The following thing works:
begintestchiendtestc, but this gives LaTeX an aneurysm: begintestcbf hiendtestc. (The error message is incomplete). I tried to debug it myself but I get hopelessly tangled up in packages that I don't understand. If you do it with tiny instead of bf you get the wildly different LaTeX error message: ! TeX capacity exceeded, sorry [input stack size=5000].

How can I squirrel away for later, not just BODY, but some sophisticated combination of BODY and other stuff, while having it robust to what's between the begin and end?

Edit: ok multiple solutions below

With unexpanded you can avoid worrying about protected@xdef.

documentclassarticle
usepackageenviron

NewEnvironexercise%
 xdefsavedexercises%
 unexpandedexpandaftersavedexercises%
 noexpandbeginprintedexercise%
 unexpandedexpandafterBODY%
 noexpandendprintedexercise%
 %

newcommandprintexercises%
 savedexercises
 gdefsavedexercises%

newcommandsavedexercises

newtheoremprintedexerciseExercise

begindocument

Here we talk about addition and show that $1+1=2$.

beginexercise
Compute $1+2$
endexercise

Here we talk about integrals.

beginexercise
Compute the following integrals:
beginitemize
item $displaystyleint_0^x e^-t^2,dt$

item $displaystyleint_1^x frace^tt,dt$, for $t>0$.
enditemize
endexercise

Now we can print the exercises.

printexercises

enddocument

I used newtheorem just for the example.

With xparse released 2019-03-05 or later:

documentclassarticle
usepackagexparse

ExplSyntaxOn

NewDocumentEnvironmentexercise+b
 
 tl_gput_right:Nn g_loisel_exercises_tl
 
 beginprintedexercise
 #1
 endprintedexercise
 
 

NewDocumentCommandprintexercises
 
 tl_use:N g_loisel_exercises_tl
 tl_gclear:N g_loisel_exercises_tl
 

tl_new:N g_loisel_exercises_tl

ExplSyntaxOff

newtheoremprintedexerciseExercise

begindocument

Here we talk about addition and show that $1+1=2$.

beginexercise
Compute $1+2$
endexercise

Here we talk about integrals.

beginexercise
Compute the following integrals:
beginitemize
item $displaystyleint_0^x e^-t^2,dt$

item $displaystyleint_1^x frace^tt,dt$, for $t>0$.
enditemize
endexercise

Now we can print the exercises.

printexercises

enddocument

If you want BODY to be saved then add stuff dynamically, you are likely best using two macros:

documentclassarticle
usepackageenviron
NewEnvirontestb%
 globalexpandafterletcsname barendcsnameBODY
 expandafterxdefcsname barplusendcsname%
 expandafternoexpandcsname barendcsname
 noexpandbf Hi
 %

begindocument

begintestb
 bfseries
 Hi
endtestb
showbarplus
enddocument

If you want to avoid using BODY you could use xparse

documentclassarticle
usepackagexparse
NewDocumentEnvironmenttestb+bexpandaftergdefcsname barendcsname#1
begindocument

begintestb
 bfseries
 Hi
endtestb
showbar
enddocument

A trick for triggering expansion is using romannumeral:

When due to romannumeral (La)TeX does gather together a sequence of digits trailed by a space as the number which it has to convert, expandable tokens get expanded.

When in the end a number is gathered together which is not positive, as the result of the conversion (La)TeX will not deliver any token at all.

Thus one can nicely (ab?)use romannumeral for triggering a lot of expansion-work and flipping-arguments-around-work as long as one ensures that in the end romannumeral will not find a positive number.

Here is a variant of egreg's answer which does with romannumeral and exchange instead of xdef and unexpanded.

documentclassarticle
usepackageenviron

newcommandexchange[2]#2#1

NewEnvironexercise%
 expandaftergdefexpandaftersavedexercisesexpandafter%
 romannumeral0expandafterexchangeexpandafterBODY%
 exchange expandaftersavedexercises
 beginprintedexercise%
 %
 endprintedexercise%
 %

newcommandprintexercises%
 savedexercises
 gdefsavedexercises%

newcommandsavedexercises

newtheoremprintedexerciseExercise

begindocument

Here we talk about addition and show that $1+1=2$.

beginexercise
Compute $1+2$
endexercise

Here we talk about integrals.

beginexercise
Compute the following integrals:
beginitemize
item $displaystyleint_0^x e^-t^2,dt$

item $displaystyleint_1^x frace^tt,dt$, for $t>0$.
enditemize
endexercise

Now we can print the exercises.

printexercises

enddocument

If you wish to wrap the name of the macro that is to be defined into csname..endcsname, i.e., if you wish to use csname savedexercisesendcsname rather than savedexercises, you can take advantage of the fact that (La)TeX does expand expandable tokens while due to csname gathering the name of a control sequence token and hereby searching for the matching endcsname:

documentclassarticle
usepackageenviron

newcommandexchange[2]#2#1

NewEnvironexercise%
 expandaftergdefcsname savedexercisesexpandafterendcsnameexpandafter%
 romannumeral0expandafterexchangeexpandafterBODY%
 exchange expandafterexpandafterexpandaftercsname savedexercisesendcsname
 beginprintedexercise%
 %
 endprintedexercise%
 %

newcommandprintexercises%
 csname savedexercisesendcsname
 expandaftergdefcsname savedexercisesendcsname%

expandafternewcommandexpandaftercsname savedexercisesendcsname

newtheoremprintedexerciseExercise

begindocument

Here we talk about addition and show that $1+1=2$.

beginexercise
Compute $1+2$
endexercise

Here we talk about integrals.

beginexercise
Compute the following integrals:
beginitemize
item $displaystyleint_0^x e^-t^2,dt$

item $displaystyleint_1^x frace^tt,dt$, for $t>0$.
enditemize
endexercise

Now we can print the exercises.

printexercises

enddocument

Be aware that with the approaches presented by now you cannot use printexercises for having exercises occur in arbitrary places. You can have exercises occur only in places of the document which in the source correspond to places behind the exercise-environments.

Perhaps an environment which does read its content under verbatim-catcode-régime for unexpanded-writing it to .aux-file in a way where from the .aux-file it gets read back under verbatim-catcode-régime also for defining a macro where scantokens will be applied to, and thus some sort of re-implementation of the label-ref-mechanism or of the tableofcontents-mechanism for verbatimized stuff might make it possible to make exercises printable throughout the entire document.

Implementing such a mechanism might be a nice challenge. But before taking that into consideration at all, exact information is needed on the intended usage and the desired "user-interface", i.e., what additional things you wish to be able to specify in which ways, etc, ...