* New language feature: with expressions.

The expression `with E1; E2' evaluates to E2 with all bindings in
  the attribute set E1 substituted.  E.g.,

    with {x = 123;}; x

  evaluates to 123.  That is, the attribute set E1 is in scope in E2.

  This is particularly useful when importing files containing lots
  definitions.  E.g., instead of

    let {
      inherit (import ./foo.nix) a b c d e f;

      body = ... a ... f ...;
    }

  we can now say

    with import ./foo.nix;

    ... a ... f ...

  I.e., we don't have to say what variables should be brought into scope.

corepkgs/buildenv/builder.pl.in

@@ -27,7 +27,7 @@ sub createLinks {
             $srcFile =~ /\/nix-support$/ ||
             $srcFile =~ /\/log$/)
         {
-            # Do noting.
+            # Do nothing.
 	}
 
         elsif (-d $srcFile) {

doc/manual/package-management.xml

@@ -17,18 +17,12 @@ available for installation.</para>
 <para>In Nix, different users can have different <quote>views</quote>
 on the set of installed applications.  That is, there might be lots of
 applications present on the system (possibly in many different
-versions), but users can have a specific selection of those
+versions), but users can have a specific selection of those active —
-active — where <quote>active</quote> just means that it appears
+where <quote>active</quote> just means that it appears in a directory
-in a directory in the user's <envar>PATH</envar>.</para>
+in the user's <envar>PATH</envar>.  Such a view on the set of
-
+installed applications is called a <emphasis>user
-<para>Such a view on the set of installed applications is called a
+environment</emphasis>, which is just a directory tree consisting of
-<emphasis>user environment</emphasis>, which is just a directory tree
+symlinks to the files of the active applications.  </para>
-consisting of symlinks to the files of the active applications.  In
-Nix, operations such as upgrading or removing components never
-overwrite or remove the files of those components, and they don't even
-touch the user environments that point to them.  Rather, they cause a
-<emphasis>new</emphasis> user environment to be constructed based on
-the old one.</para>
 
 <para>Components are installed from a set of <emphasis>Nix
 expressions</emphasis> that tell Nix how to build those components,
@@ -168,7 +162,21 @@ set.</para></footnote></para>
 
 <sect1><title>Profiles</title>
 
-<para>Bla</para>
+<para>In Nix, operations such as upgrading or removing components
+never overwrite or remove the files of those components, and they
+don't even touch the user environments that point to them.  Rather,
+they cause a <emphasis>new</emphasis> user environment to be
+constructed based on the old one.  This is illustrated in Figure
+bla.</para>
+
+<figure><title>User environments</title>
+  <mediaobject>
+    <imageobject>
+      <imagedata fileref='figures/userenv-frame3.png' format='PNG' />
+    </imageobject>
+  </mediaobject>
+</figure>
+
 
 </sect1>
 

src/libexpr/eval.cc

@@ -230,7 +230,7 @@ Expr evalExpr2(EvalState & state, Expr e)
             try {
                 return evalExpr(state, substArgs(e4, formals, e2));
             } catch (Error & e) {
-                throw Error(format("while evaluating function at %1%:\n%2%")
+                throw Error(format("while evaluating the function at %1%:\n%2%")
                     % showPos(pos) % e.msg());
             }
         }
@@ -241,7 +241,7 @@ Expr evalExpr2(EvalState & state, Expr e)
                 subs.set(name, e2);
                 return evalExpr(state, substitute(subs, e4));
             } catch (Error & e) {
-                throw Error(format("while evaluating function at %1%:\n%2%")
+                throw Error(format("while evaluating the function at %1%:\n%2%")
                     % showPos(pos) % e.msg());
             }
         }
@@ -258,7 +258,7 @@ Expr evalExpr2(EvalState & state, Expr e)
         try {
             return evalExpr(state, a);
         } catch (Error & e) {
-            throw Error(format("while evaluating attribute `%1%' at %2%:\n%3%")
+            throw Error(format("while evaluating the attribute `%1%' at %2%:\n%3%")
                 % s1 % showPos(pos) % e.msg());
         }
     }
@@ -283,6 +283,26 @@ Expr evalExpr2(EvalState & state, Expr e)
         return evalExpr(state, e2);
     }
 
+    /* Withs. */
+    if (atMatch(m, e) >> "With" >> e1 >> e2 >> pos) {
+        ATermMap attrs;
+        try {
+            e1 = evalExpr(state, e1);
+            queryAllAttrs(e1, attrs);
+        } catch (Error & e) {
+            throw Error(format("while evaluating the `with' definitions at %1%:\n%2%")
+                % showPos(pos) % e.msg());
+        }
+        try {
+            e2 = substitute(attrs, e2);
+            checkVarDefs(state.primOps, e2);
+            return evalExpr(state, e2);
+        } catch (Error & e) {
+            throw Error(format("while evaluating the `with' body at %1%:\n%2%")
+                % showPos(pos) % e.msg());
+        } 
+    }
+
     /* Generic equality. */
     if (atMatch(m, e) >> "OpEq" >> e1 >> e2)
         return makeBool(evalExpr(state, e1) == evalExpr(state, e2));
@@ -357,7 +377,7 @@ Expr evalFile(EvalState & state, const Path & path)
     try {
         return evalExpr(state, e);
     } catch (Error & e) {
-        throw Error(format("while evaluating file `%1%':\n%2%")
+        throw Error(format("while evaluating the file `%1%':\n%2%")
             % path % e.msg());
     }
 }

src/libexpr/lexer.l

@@ -48,6 +48,7 @@ if          { return IF; }
 then        { return THEN; }
 else        { return ELSE; }
 assert      { return ASSERT; }
+with        { return WITH; }
 let         { return LET; }
 rec         { return REC; }
 inherit     { return INHERIT; }

src/libexpr/nixexpr.cc

@@ -296,7 +296,7 @@ void checkVarDefs(const ATermMap & defs, Expr e)
     ATMatcher m;
     ATerm name;
     ATermList formals;
-    ATerm body;
+    ATerm with, body;
     ATermList rbnds, nrbnds;
 
     if (atMatch(m, e) >> "Var" >> name) {
@@ -340,6 +340,13 @@ void checkVarDefs(const ATermMap & defs, Expr e)
         }
         checkVarDefs(defs2, (ATerm) rbnds);
     }
+
+    else if (atMatch(m, e) >> "With" >> with >> body) {
+        /* We can't check the body without evaluating the definitions
+           (which is an arbitrary expression), so we don't do that
+           here but only when actually evaluating the `with'. */
+        checkVarDefs(defs, with);
+    }
     
     else if (ATgetType(e) == AT_APPL) {
         int arity = ATgetArity(ATgetAFun(e));

src/libexpr/parser.y

@@ -41,11 +41,11 @@ ATerm makePos(YYLTYPE * loc, void * data)
   ATermList ts;
 }
 
-%type <t> start expr expr_function expr_assert expr_if expr_op
+%type <t> start expr expr_function expr_if expr_op
 %type <t> expr_app expr_select expr_simple bind inheritsrc formal
 %type <ts> binds ids expr_list formals
 %token <t> ID INT STR PATH URI
-%token IF THEN ELSE ASSERT LET REC INHERIT EQ NEQ AND OR IMPL
+%token IF THEN ELSE ASSERT WITH LET REC INHERIT EQ NEQ AND OR IMPL
 
 %nonassoc IMPL
 %left OR
@@ -67,12 +67,10 @@ expr_function
     { $$ = ATmake("Function(<term>, <term>, <term>)", $2, $5, CUR_POS); }
   | ID ':' expr_function
     { $$ = ATmake("Function1(<term>, <term>, <term>)", $1, $3, CUR_POS); }
-  | expr_assert
+  | ASSERT expr ';' expr_function
-  ;
-
-expr_assert
-  : ASSERT expr ';' expr_assert
     { $$ = ATmake("Assert(<term>, <term>, <term>)", $2, $4, CUR_POS); }
+  | WITH expr ';' expr_function
+    { $$ = ATmake("With(<term>, <term>, <term>)", $2, $4, CUR_POS); }
   | expr_if
   ;
 

src/libexpr/primops.cc

@@ -205,7 +205,7 @@ static Expr primDerivation(EvalState & state, const ATermVector & _args)
         try {
             processBinding(state, value, ne, ss);
         } catch (Error & e) {
-            throw Error(format("while processing derivation attribute `%1%' at %2%:\n%3%")
+            throw Error(format("while processing the derivation attribute `%1%' at %2%:\n%3%")
                 % key % showPos(pos) % e.msg());
         }