* Refactoring: put the Nix expression evaluator in its own library so

that it can be used by multiple programs.

src/libexpr/nixexpr.hh

@@ -0,0 +1,75 @@
+#ifndef __NIXEXPR_H
+#define __NIXEXPR_H
+
+#include <map>
+
+#include <aterm2.h>
+
+#include "util.hh"
+
+
+/* Nix expressions are represented as ATerms.  The maximal sharing
+   property of the ATerm library allows us to implement caching of
+   normals forms efficiently. */
+typedef ATerm Expr;
+
+
+/* Mappings from ATerms to ATerms.  This is just a wrapper around
+   ATerm tables. */
+class ATermMap
+{
+private:
+    unsigned int maxLoadPct;
+    ATermTable table;
+    
+public:
+    ATermMap(unsigned int initialSize = 16, unsigned int maxLoadPct = 75);
+    ATermMap(const ATermMap & map);
+    ~ATermMap();
+
+    void set(ATerm key, ATerm value);
+    void set(const string & key, ATerm value);
+
+    ATerm get(ATerm key) const;
+    ATerm get(const string & key) const;
+
+    void remove(ATerm key);
+    void remove(const string & key);
+
+    ATermList keys() const;
+};
+
+
+/* Convert a string to an ATerm (i.e., a quoted nullary function
+   applicaton). */
+ATerm string2ATerm(const string & s);
+string aterm2String(ATerm t);
+
+/* Generic bottomup traversal over ATerms.  The traversal first
+   recursively descends into subterms, and then applies the given term
+   function to the resulting term. */
+struct TermFun
+{
+    virtual ATerm operator () (ATerm e) = 0;
+};
+ATerm bottomupRewrite(TermFun & f, ATerm e);
+
+/* Query all attributes in an attribute set expression.  The
+   expression must be in normal form. */
+void queryAllAttrs(Expr e, ATermMap & attrs);
+
+/* Query a specific attribute from an attribute set expression.  The
+   expression must be in normal form. */
+Expr queryAttr(Expr e, const string & name);
+
+/* Create an attribute set expression from an Attrs value. */
+Expr makeAttrs(const ATermMap & attrs);
+
+/* Perform a set of substitutions on an expression. */
+Expr substitute(const ATermMap & subs, Expr e);
+
+/* Create an expression representing a boolean. */
+Expr makeBool(bool b);
+
+
+#endif /* !__NIXEXPR_H */