[erlang-questions] better ct_expand and some intricate problems

Ulf Wiger <>
Sat Jan 28 17:09:39 CET 2012


I have made some improvements to ct_expand

https://github.com/uwiger/parse_trans/blob/master/doc/ct_expand.md

It used to be that it could only expand simple expressions. Now, it can handle calls to local functions, which are then interpreted. It also handles things like fun foo/1, by fetching the function definition and inlining it as a "regular" fun.

Unfortunately, I failed to bring it up to the level that I aspired to, namely to improve some of Tony Rogvall's code (not yet released). The problem there was that his code called functions that stored funs in a dict. Doing this at compile-time, the funs would - at best - end up being interpreted. As it was, I didn't even get that far, since funs created by one interpreted function couldn't be passed as arguments to another interpreted function.

The culprits are erl_eval, which unconditionally converts the return values into regular terms, and erl_parse:abstract/1, which fails to create abstract expressions from funs. Thus, the conversion to regular terms is not reversible in this case.

In https://github.com/uwiger/parse_trans/blob/master/examples/ct_expand_test.erl#L15, an example is given of what kind of things can now be done with ct_expand.

I added a trace option, so that it would be easier to debug complex expansions:

Eshell V5.8.4  (abort with ^G)
1> c(ct_expand_test,[{ct_expand_trace,[r]}]).
ct_expand (27): call zip([1,2],[a,b])
ct_expand (27): call zip([2],[b])
ct_expand (27): call zip([],[])
ct_expand (27): returned from zip/2: []
ct_expand (27): returned from zip/2: [{{2},{b}}]
ct_expand (27): returned from zip/2: [{{1},{a}},{{2},{b}}]
Pretty-printed in "./ct_expand_test.xfm"

(Not perfect: the functions that were inlined don't show up in the trace.)

I welcome ideas on how to get around the above limitation with reasonable effort (I know I could make my own evaluator, but that seems like too high a price).

Oh, the error reporting from parse_trans has been greatly improved. It now detects parse_errors immediately, and aborts the transform, and also reports transform errors in a less noisy way. Let me know if you have issues with it.

BR,
Ulf W


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