On Thu, Sep 24, 2009 at 12:51, Eelco Dolstra <e.dols...@tudelft.nl> wrote: > Hi all, > > Currently there are many places in the Nixpkgs and NixOS expressions where we > have to select a possibly missing attribute from a set, e.g. > > attrByPath ["extraModules"] [] mainCfg > > getConfig ["python" "db4Support"] true
set ? attr && set.attr > > Writing `attrByPath ["a" "b" "c"] default as' is rather verbose and ugly > compared to selecting an attribute without a default, i.e. `as.a.b.c'. > > Since this is fairly common, how about some special syntax for supplying a > default value to the "." selection operator? I propose the optional "or" > keyword following an attribute selection "x.<attrpath>". E.g. Arguments are in: http://www.mail-archive.com/nix-dev@cs.uu.nl/msg02725.html 1/ The "or" operator: * mainCfg.extraModules or [] * config.python.db4Support or true * set.attr or false scope of the default value, * as.a.b.c or default * (as.a).b.c or default * (as.a.b).c or default Cons: - Feel like with miss a "and" operator. - The scoping is not intuitive. - " or false " can be miss-leading. Pros: - Small operator. 2/ The curly braces with question mark: * mainCfg.{extraModules ? []} * config.{python.db4Support ? true} * set.{attr ? false} scope of the default value, * as.{a.b.c ? default} * as.a.{b.c ? default} * as.a.b.{c ? default} This operator can be derived into other like: a.(b.c :- d) / a.(b.c ?= d) Cons: - Can be interpreted as a function argument with a default value (don't miss the leading ".") - Large operator with its braces. - Can be confused with the hasAttr operator "?". Pros: - Scoping is clear 3/ Merging proposals * mainCfg.extraModules ?: [] * config.python.db4Support ?: true * set.attr ?: false scope of the default value, * as.a.b.c ?: default * (as.a.b.c ?: default) # equivalent to the previous one * as.(a.b.c ?: default) # equivalent to the previous one * as.a.(b.c ?: default) * as.a.b.(c ?: default) "?:" is used instead of "or" because "?" is used for testing if an attribute exists and the common usage of the ternary operator in other C like languages is "cond ? if-true : if-false" which would mean in our case: if-has ?: if-not Cons: - "?:" may lead to questions about "cond ? true : false". Pros: - small operator - clear scoping rules To avoid the "Cons", we can use "?!". This operator can be named the " has - not " operator. -- Nicolas Pierron http://www.linkedin.com/in/nicolasbpierron - http://nbp.name/ Andrew S. Tanenbaum - Never underestimate the bandwith of a wagon full of tapes. _______________________________________________ nix-dev mailing list nix-dev@cs.uu.nl https://mail.cs.uu.nl/mailman/listinfo/nix-dev
