* Another experimental feature: a way to truly override attributes in
a rec. This will be very useful to allow end-user customisation of all-packages.nix, for instance globally overriding GCC or some other dependency. The // operator doesn't cut it: you could replace the "gcc" attribute, but all other attributes would continue to reference the original value due to the substitution semantics of rec. The syntax is a bit hacky but this is to allow backwards compatibility.
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@ -170,16 +170,18 @@ static Expr substArgs(EvalState & state,
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to attributes substituted with selection expressions on the
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original set. E.g., e = `rec {x = f x y; y = x;}' becomes `{x = f
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(e.x) (e.y); y = e.x;}'. */
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LocalNoInline(ATerm expandRec(ATerm e, ATermList rbnds, ATermList nrbnds))
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LocalNoInline(ATerm expandRec(EvalState & state, ATerm e, ATermList rbnds, ATermList nrbnds))
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{
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ATerm name;
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Expr e2;
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Pos pos;
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Expr eOverrides = 0;
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/* Create the substitution list. */
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ATermMap subs(ATgetLength(rbnds) + ATgetLength(nrbnds));
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for (ATermIterator i(rbnds); i; ++i) {
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if (!matchBind(*i, name, e2, pos)) abort(); /* can't happen */
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if (name == sOverrides) eOverrides = e2;
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subs.set(name, makeSelect(e, name));
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}
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for (ATermIterator i(nrbnds); i; ++i) {
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@ -187,6 +189,22 @@ LocalNoInline(ATerm expandRec(ATerm e, ATermList rbnds, ATermList nrbnds))
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subs.set(name, e2);
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}
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/* If the rec contains an attribute called `__overrides', then
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evaluate it, and add the attributes in that set to the rec.
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This allows overriding of recursive attributes, which is
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otherwise not possible. (You can use the // operator to
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replace an attribute, but other attributes in the rec will
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still reference the original value, because that value has been
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substituted into the bodies of the other attributes. Hence we
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need __overrides.) */
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ATermMap overrides;
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if (eOverrides) {
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eOverrides = evalExpr(state, eOverrides);
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queryAllAttrs(overrides, overrides, false);
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foreach (ATermMap::const_iterator, i, overrides)
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subs.set(i->key, i->value);
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}
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Substitution subs_(0, &subs);
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/* Create the non-recursive set. */
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@ -196,6 +214,10 @@ LocalNoInline(ATerm expandRec(ATerm e, ATermList rbnds, ATermList nrbnds))
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as.set(name, makeAttrRHS(substitute(subs_, e2), pos));
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}
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if (eOverrides)
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foreach (ATermMap::const_iterator, i, overrides)
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as.set(i->key, makeAttrRHS(i->value, makeNoPos()));
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/* Copy the non-recursive bindings. !!! inefficient */
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for (ATermIterator i(nrbnds); i; ++i) {
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if (!matchBind(*i, name, e2, pos)) abort(); /* can't happen */
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@ -667,7 +689,7 @@ Expr evalExpr2(EvalState & state, Expr e)
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/* Mutually recursive sets. */
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ATermList rbnds, nrbnds;
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if (matchRec(e, rbnds, nrbnds))
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return expandRec(e, rbnds, nrbnds);
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return expandRec(state, e, rbnds, nrbnds);
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/* Conditionals. */
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if (matchIf(e, e1, e2, e3))
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@ -93,3 +93,4 @@ AttrRHS | Expr Pos | ATerm |
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eTrue = makeBool(makeTrue())
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eFalse = makeBool(makeFalse())
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sOverrides = toATerm("__overrides")
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