* 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.

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
-  ;
-
-expr_assert
-  : ASSERT expr ';' expr_assert
+  | ASSERT expr ';' expr_function
     { $$ = ATmake("Assert(<term>, <term>, <term>)", $2, $4, CUR_POS); }
+  | WITH expr ';' expr_function
+    { $$ = 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());
         }