diff --git a/edify/expr.h b/edify/expr.h
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+++ b/edify/expr.h
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+/*
+ * Copyright (C) 2009 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef _EXPRESSION_H
+#define _EXPRESSION_H
+
+#include <unistd.h>
+
+#include "yydefs.h"
+
+#define MAX_STRING_LEN 1024
+
+typedef struct Expr Expr;
+
+typedef struct {
+    // Optional pointer to app-specific data; the core of edify never
+    // uses this value.
+    void* cookie;
+
+    // The source of the original script.  Must be NULL-terminated,
+    // and in writable memory (Evaluate may make temporary changes to
+    // it but will restore it when done).
+    char* script;
+
+    // The error message (if any) returned if the evaluation aborts.
+    // Should be NULL initially, will be either NULL or a malloc'd
+    // pointer after Evaluate() returns.
+    char* errmsg;
+} State;
+
+#define VAL_STRING  1  // data will be NULL-terminated; size doesn't count null
+#define VAL_BLOB    2
+
+typedef struct {
+    int type;
+    ssize_t size;
+    char* data;
+} Value;
+
+typedef Value* (*Function)(const char* name, State* state,
+                           int argc, Expr* argv[]);
+
+struct Expr {
+    Function fn;
+    char* name;
+    int argc;
+    Expr** argv;
+    int start, end;
+};
+
+// Take one of the Expr*s passed to the function as an argument,
+// evaluate it, return the resulting Value.  The caller takes
+// ownership of the returned Value.
+Value* EvaluateValue(State* state, Expr* expr);
+
+// Take one of the Expr*s passed to the function as an argument,
+// evaluate it, assert that it is a string, and return the resulting
+// char*.  The caller takes ownership of the returned char*.  This is
+// a convenience function for older functions that want to deal only
+// with strings.
+char* Evaluate(State* state, Expr* expr);
+
+// Glue to make an Expr out of a literal.
+Value* Literal(const char* name, State* state, int argc, Expr* argv[]);
+
+// Functions corresponding to various syntactic sugar operators.
+// ("concat" is also available as a builtin function, to concatenate
+// more than two strings.)
+Value* ConcatFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* LogicalAndFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* LogicalOrFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* LogicalNotFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* SubstringFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* EqualityFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* InequalityFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* SequenceFn(const char* name, State* state, int argc, Expr* argv[]);
+
+// Convenience function for building expressions with a fixed number
+// of arguments.
+Expr* Build(Function fn, YYLTYPE loc, int count, ...);
+
+// Global builtins, registered by RegisterBuiltins().
+Value* IfElseFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* AssertFn(const char* name, State* state, int argc, Expr* argv[]);
+Value* AbortFn(const char* name, State* state, int argc, Expr* argv[]);
+
+
+// For setting and getting the global error string (when returning
+// NULL from a function).
+void SetError(const char* message);  // makes a copy
+const char* GetError();              // retains ownership
+void ClearError();
+
+
+typedef struct {
+  const char* name;
+  Function fn;
+} NamedFunction;
+
+// Register a new function.  The same Function may be registered under
+// multiple names, but a given name should only be used once.
+void RegisterFunction(const char* name, Function fn);
+
+// Register all the builtins.
+void RegisterBuiltins();
+
+// Call this after all calls to RegisterFunction() but before parsing
+// any scripts to finish building the function table.
+void FinishRegistration();
+
+// Find the Function for a given name; return NULL if no such function
+// exists.
+Function FindFunction(const char* name);
+
+
+// --- convenience functions for use in functions ---
+
+// Evaluate the expressions in argv, giving 'count' char* (the ... is
+// zero or more char** to put them in).  If any expression evaluates
+// to NULL, free the rest and return -1.  Return 0 on success.
+int ReadArgs(State* state, Expr* argv[], int count, ...);
+
+// Evaluate the expressions in argv, giving 'count' Value* (the ... is
+// zero or more Value** to put them in).  If any expression evaluates
+// to NULL, free the rest and return -1.  Return 0 on success.
+int ReadValueArgs(State* state, Expr* argv[], int count, ...);
+
+// Evaluate the expressions in argv, returning an array of char*
+// results.  If any evaluate to NULL, free the rest and return NULL.
+// The caller is responsible for freeing the returned array and the
+// strings it contains.
+char** ReadVarArgs(State* state, int argc, Expr* argv[]);
+
+// Evaluate the expressions in argv, returning an array of Value*
+// results.  If any evaluate to NULL, free the rest and return NULL.
+// The caller is responsible for freeing the returned array and the
+// Values it contains.
+Value** ReadValueVarArgs(State* state, int argc, Expr* argv[]);
+
+// Use printf-style arguments to compose an error message to put into
+// *state.  Returns NULL.
+Value* ErrorAbort(State* state, char* format, ...);
+
+// Wrap a string into a Value, taking ownership of the string.
+Value* StringValue(char* str);
+
+// Free a Value object.
+void FreeValue(Value* v);
+
+#endif  // _EXPRESSION_H