bug#34999: Record special field abstraction leakage

[Leo Prikler]

Hi Ludo,

I think I have found out why users see the thunked fields as below.
Am Dienstag, den 26.03.2019, 10:38 +0100 schrieb Ludovic Courtès:
> The changes I made in version-control.scm and gnucash.scm in commit
> e6301fb76d0a8d931ece2e18d197e3c2cc53fc6c revealed an abstraction
> leakage
> I wasn’t aware of: there’s a pattern where users “see” that thunked
> fields are thunked:
> 
>   (package
>     ;; …
>     (inputs …)
>     (arguments `(foo bar ,(inputs) …)))  ;<- here ‘inputs’ is seen as
> a thunk
The issue is that for constructing the records, we let*-bind the field
names to their values before calling the constructor.  In these let*-
bindings the fields are already wrapped, e.g. inputs will be bound to
the value that the record field inputs will have, not to the raw value.

I've attached a patch to fix this issue as well as a MWE to try it out.
I'm not sure about the broader semantics of this patch, though.  I fear
that exposing raw values through let-binding probably eliminates the
delayed/thunked nature of said fields in some ways.  WDYT?

0001-guix-records-let-bind-raw-values-wrap-them-in-constr.patch
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34999-mwe.scm
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