diff --git a/src/runtime/c/Makefile.am b/src/runtime/c/Makefile.am
index a9fd75ecb..75bd0a253 100644
--- a/src/runtime/c/Makefile.am
+++ b/src/runtime/c/Makefile.am
@@ -80,6 +80,7 @@ libpgf_la_SOURCES = \
 	pgf/linearizer.c \
 	pgf/typechecker.c \
 	pgf/reasoner.c \
+	pgf/evaluator.c \
 	pgf/printer.c \
 	pgf/graphviz.c \
 	pgf/pgf.c \
diff --git a/src/runtime/c/pgf/evaluator.c b/src/runtime/c/pgf/evaluator.c
new file mode 100644
index 000000000..7c4598a86
--- /dev/null
+++ b/src/runtime/c/pgf/evaluator.c
@@ -0,0 +1,443 @@
+#include "pgf/pgf.h"
+#include "pgf/data.h"
+
+typedef struct PgfEnv PgfEnv;
+typedef struct PgfClosure PgfClosure;
+typedef struct PgfEvalState PgfEvalState;
+
+struct PgfEnv {
+	PgfEnv* next;
+	PgfClosure* closure;
+};
+
+struct PgfClosure {
+	PgfClosure* (*code)(PgfEvalState* state, PgfClosure* val);
+};
+
+typedef struct {
+	PgfClosure header;
+	PgfEnv*  env;
+	PgfExpr  expr;
+} PgfExprThunk;
+
+typedef struct {
+	PgfClosure header;
+	PgfClosure* val;
+} PgfIndirection;
+
+typedef struct {
+	PgfClosure header;
+	PgfAbsFun* absfun;
+	size_t n_args;
+	PgfClosure* args[];
+} PgfValue;
+
+typedef struct {
+	PgfClosure header;
+	int level;
+	size_t n_args;
+	PgfClosure* args[];
+} PgfValueGen;
+
+typedef struct {
+	PgfClosure header;
+	PgfEnv*  env;
+	PgfMetaId id;
+	size_t n_args;
+	PgfClosure* args[];
+} PgfValueMeta;
+
+typedef struct {
+	PgfClosure header;
+	PgfLiteral lit;
+} PgfValueLit;
+
+struct PgfEvalState {
+	PgfPGF* pgf;
+	GuPool* pool;
+	GuExn* err;
+	GuBuf* stack;
+};
+
+static PgfClosure*
+pgf_evaluate_indirection(PgfEvalState* state, PgfClosure* closure)
+{
+	PgfIndirection* indir = (PgfIndirection*) closure;
+	return indir->val;
+}
+
+static PgfClosure*
+pgf_evaluate_value(PgfEvalState* state, PgfClosure* closure)
+{
+	PgfValue* val = (PgfValue*) closure;
+
+	size_t n_args = val->n_args + gu_buf_length(state->stack);
+	PgfValue* new_val =
+		gu_new_flex(state->pool, PgfValue, args, n_args);
+	new_val->header.code = pgf_evaluate_value;
+	new_val->absfun = val->absfun;
+	new_val->n_args = n_args;
+	
+	size_t i = 0;
+	while (i < val->n_args) {
+		new_val->args[i] = val->args[i];
+		i++;
+	}
+	while (i < n_args) {
+		val->args[i] = gu_buf_pop(state->stack, PgfClosure*);
+		i++;
+	}
+
+	return &new_val->header;
+}
+
+static PgfClosure*
+pgf_evaluate_value_gen(PgfEvalState* state, PgfClosure* closure)
+{
+	PgfValueGen* val = (PgfValueGen*) closure;
+
+	size_t n_args = val->n_args + gu_buf_length(state->stack);
+	PgfValueGen* new_val =
+		gu_new_flex(state->pool, PgfValueGen, args, n_args);
+	new_val->header.code = pgf_evaluate_value_gen;
+	new_val->level = val->level;
+	new_val->n_args = n_args;
+	
+	size_t i = 0;
+	while (i < val->n_args) {
+		new_val->args[i] = val->args[i];
+		i++;
+	}
+	while (i < n_args) {
+		new_val->args[i] = gu_buf_pop(state->stack, PgfClosure*);
+		i++;
+	}
+
+	return &new_val->header;
+}
+
+static PgfClosure*
+pgf_evaluate_value_meta(PgfEvalState* state, PgfClosure* closure)
+{
+	PgfValueMeta* val = (PgfValueMeta*) closure;
+
+	size_t n_args = val->n_args + gu_buf_length(state->stack);
+	PgfValueMeta* new_val =
+		gu_new_flex(state->pool, PgfValueMeta, args, n_args);
+	new_val->header.code = pgf_evaluate_value_meta;
+	new_val->id = val->id;
+	new_val->n_args = n_args;
+	
+	size_t i = 0;
+	while (i < val->n_args) {
+		new_val->args[i] = val->args[i];
+		i++;
+	}
+	while (i < n_args) {
+		val->args[i] = gu_buf_pop(state->stack, PgfClosure*);
+		i++;
+	}
+
+	return &new_val->header;
+}
+
+static PgfClosure*
+pgf_evaluate_value_lit(PgfEvalState* state, PgfClosure* closure)
+{
+	return closure;
+}
+
+static PgfClosure*
+pgf_evaluate_expr_thunk(PgfEvalState* state, PgfClosure* closure)
+{
+	PgfExprThunk* thunk = (PgfExprThunk*) closure;
+	PgfEnv* env  = thunk->env;
+	PgfExpr expr = thunk->expr;
+
+	for (;;) {
+		GuVariantInfo ei = gu_variant_open(expr);
+		switch (ei.tag) {
+		case PGF_EXPR_ABS: {
+			PgfExprAbs* eabs = ei.data;
+
+			if (gu_buf_length(state->stack) > 0) {
+				PgfEnv* new_env  = gu_new(PgfEnv, state->pool);
+				new_env->next    = env;
+				new_env->closure = gu_buf_pop(state->stack, PgfClosure*);
+
+				env  = new_env;
+				expr = eabs->body;
+			} else {
+				thunk->expr = expr;
+				return closure;
+			}
+			break;
+		}
+		case PGF_EXPR_APP: {
+			PgfExprApp* eapp = ei.data;
+			PgfExprThunk* thunk = 
+				gu_new(PgfExprThunk, state->pool);
+			thunk->header.code = pgf_evaluate_expr_thunk;
+			thunk->env  = env;
+			thunk->expr = eapp->arg;
+			gu_buf_push(state->stack, PgfClosure*, &thunk->header);
+			expr = eapp->fun;
+			break;
+		}
+		case PGF_EXPR_LIT: {
+			PgfExprLit* elit = ei.data;
+
+			if (gu_buf_length(state->stack) > 0) {
+				GuExnData* err_data = gu_raise(state->err, PgfExn);
+				if (err_data) {
+					err_data->data = "found literal of function type";
+				}
+				return NULL;
+			}
+
+			PgfValueLit* val = (PgfValueLit*) closure;
+			val->header.code = pgf_evaluate_value_lit;
+			val->lit = elit->lit;
+			return &val->header;
+		}
+		case PGF_EXPR_META: {
+			PgfExprMeta* emeta = ei.data;
+
+			size_t n_args = gu_buf_length(state->stack);
+
+			PgfValueMeta* val =
+				gu_new_flex(state->pool, PgfValueMeta, args, n_args);
+			val->header.code = pgf_evaluate_value_meta;
+			val->id = emeta->id;
+			val->n_args  = n_args;
+			for (size_t i = 0; i < n_args; i++) {
+				val->args[i] = gu_buf_pop(state->stack, PgfClosure*);
+			}
+
+			PgfIndirection* indir = (PgfIndirection*) closure;
+			indir->header.code = pgf_evaluate_indirection;
+			indir->val         = &val->header;
+
+			return &val->header;
+		}
+		case PGF_EXPR_FUN: {
+			PgfExprFun* efun = ei.data;
+
+			PgfAbsFun* absfun =
+				gu_map_get(state->pgf->abstract.funs, efun->fun, PgfAbsFun*);
+			if (absfun == NULL) {
+				GuExnData* err_data = gu_raise(state->err, PgfExn);
+				if (err_data) {
+					err_data->data = (char* const) 
+						gu_format_string(err_data->pool,
+						                 "Unknown function: %s",
+						                 efun->fun);
+				}
+				return NULL;
+			}
+
+			size_t n_args = gu_buf_length(state->stack);
+
+			PgfValue* val =
+				gu_new_flex(state->pool, PgfValue, args, n_args);
+			val->header.code = pgf_evaluate_value;
+			val->absfun = absfun;
+			val->n_args = n_args;
+			for (size_t i = 0; i < n_args; i++) {
+				val->args[i] = gu_buf_pop(state->stack, PgfClosure*);
+			}
+
+			PgfIndirection* indir = (PgfIndirection*) closure;
+			indir->header.code = pgf_evaluate_indirection;
+			indir->val         = &val->header;
+
+			return &val->header;
+		}
+		case PGF_EXPR_VAR: {
+			PgfExprVar* evar = ei.data;
+			PgfEnv* tmp_env = env;
+			size_t i = evar->var;
+			while (i > 0) {
+				tmp_env = tmp_env->next;
+				if (tmp_env == NULL) {
+					GuExnData* err_data = gu_raise(state->err, PgfExn);
+					if (err_data) {
+						err_data->data = "invalid de Bruijn index";
+					}
+					return NULL;
+				}
+				i--;
+			}
+
+			PgfClosure* val = 
+				tmp_env->closure->code(state, tmp_env->closure);
+
+			PgfIndirection* indir = (PgfIndirection*) closure;
+			indir->header.code = pgf_evaluate_indirection;
+			indir->val         = val;
+
+			return val;
+		}
+		case PGF_EXPR_TYPED: {
+			PgfExprTyped* etyped = ei.data;
+			expr = etyped->expr;
+			break;
+		}
+		case PGF_EXPR_IMPL_ARG: {
+			PgfExprImplArg* eimpl = ei.data;
+			expr = eimpl->expr;
+			break;
+		}
+		default:
+			gu_impossible();
+		}
+	}
+}
+
+static PgfExpr
+pgf_value2expr(PgfEvalState* state, int level, PgfClosure* clos, GuPool* pool)
+{
+	clos = clos->code(state, clos);
+	if (clos == NULL)
+		return gu_null_variant;
+
+	PgfExpr expr = gu_null_variant;
+	size_t n_args = 0;
+	PgfClosure** args;
+
+	if (clos->code == pgf_evaluate_value) {
+		PgfValue* val = (PgfValue*) clos;
+
+		expr   = val->absfun->ep.expr;
+		n_args = val->n_args;
+		args   = val->args;
+	} else if (clos->code == pgf_evaluate_value_gen) {
+		PgfValueGen* val = (PgfValueGen*) clos;
+
+		PgfExprVar *evar =
+			gu_new_variant(PGF_EXPR_VAR,
+						   PgfExprVar,
+						   &expr, pool);
+		evar->var = level - val->level - 1;
+
+		n_args = val->n_args;
+		args   = val->args;
+	} else if (clos->code == pgf_evaluate_value_meta) {
+		PgfValueMeta* val = (PgfValueMeta*) clos;
+
+		PgfExprMeta *emeta =
+			gu_new_variant(PGF_EXPR_META,
+						   PgfExprMeta,
+						   &expr, pool);
+		emeta->id = val->id;
+
+		n_args = val->n_args;
+		args   = val->args;
+	} else if (clos->code == pgf_evaluate_value_lit) {
+		PgfValueLit* val = (PgfValueLit*) clos;
+
+		PgfExprLit *elit =
+			gu_new_variant(PGF_EXPR_LIT,
+						   PgfExprLit,
+						   &expr, pool);
+						   
+		GuVariantInfo i = gu_variant_open(val->lit);
+		switch (i.tag) {
+		case PGF_LITERAL_STR: {
+			PgfLiteralStr* lstr = i.data;
+
+			PgfLiteralStr* new_lstr =
+				gu_new_flex_variant(PGF_LITERAL_STR,
+									PgfLiteralStr,
+									val, strlen(lstr->val)+1,
+									&elit->lit, pool);
+			strcpy(new_lstr->val, lstr->val);
+			break;
+		}
+		case PGF_LITERAL_INT: {
+			PgfLiteralInt* lint = i.data;
+
+			PgfLiteralInt* new_lint =
+				gu_new_variant(PGF_LITERAL_INT,
+							   PgfLiteralInt,
+							   &elit->lit, pool);
+			new_lint->val = lint->val;
+			break;
+		}
+		case PGF_LITERAL_FLT: {
+			PgfLiteralFlt* lflt = i.data;
+
+			PgfLiteralFlt* new_lflt =
+				gu_new_variant(PGF_LITERAL_FLT,
+							   PgfLiteralFlt,
+							   &elit->lit, pool);
+			new_lflt->val = lflt->val;
+			break;
+		}
+		default:
+			gu_impossible();
+		}
+	} else {
+		PgfExprThunk *old_thunk = (PgfExprThunk*) clos;
+		PgfExprAbs *old_eabs = gu_variant_open(old_thunk->expr).data;
+
+		PgfValueGen* gen =
+			gu_new(PgfValueGen, state->pool);
+		gen->header.code = pgf_evaluate_value_gen;
+		gen->level  = level;
+		gen->n_args = 0;
+
+		PgfEnv* new_env  = gu_new(PgfEnv, state->pool);
+		new_env->next    = old_thunk->env;
+		new_env->closure = &gen->header;
+
+		PgfExprThunk* new_thunk =
+			gu_new(PgfExprThunk, state->pool);
+		new_thunk->header.code = pgf_evaluate_expr_thunk;
+		new_thunk->env  = new_env;
+		new_thunk->expr = old_eabs->body;
+
+		PgfExprAbs *eabs =
+			gu_new_variant(PGF_EXPR_ABS,
+						   PgfExprAbs,
+						   &expr, pool);
+		eabs->bind_type = old_eabs->bind_type;
+		eabs->id = gu_format_string(pool, "v%d", level);
+		eabs->body = pgf_value2expr(state, level+1, &new_thunk->header, pool);
+	}
+
+	for (size_t i = 0; i < n_args; i++) {
+		PgfExpr fun = expr;
+		PgfExpr arg = 
+			pgf_value2expr(state, level, args[i], pool);
+		if (gu_variant_is_null(arg))
+			return gu_null_variant;
+
+		PgfExprApp* e =
+			gu_new_variant(PGF_EXPR_APP,
+						   PgfExprApp,
+						   &expr, pool);
+		e->fun = fun;
+		e->arg = arg;
+	}
+
+	return expr;
+}
+
+PgfExpr
+pgf_compute(PgfPGF* pgf, PgfExpr expr, GuExn* err, GuPool* pool, GuPool* out_pool)
+{
+	PgfEvalState* state = gu_new(PgfEvalState, pool);
+	state->pgf   = pgf;
+	state->pool  = pool;
+	state->err   = err;
+	state->stack = gu_new_buf(PgfClosure*, pool);
+
+	PgfExprThunk* thunk =
+		gu_new(PgfExprThunk, pool);
+	thunk->header.code = pgf_evaluate_expr_thunk;
+	thunk->env  = NULL;
+	thunk->expr = expr;
+
+	return pgf_value2expr(state, 0, &thunk->header, out_pool);
+}
diff --git a/src/runtime/c/pgf/pgf.h b/src/runtime/c/pgf/pgf.h
index 7cddbbcae..bc9fb7d99 100644
--- a/src/runtime/c/pgf/pgf.h
+++ b/src/runtime/c/pgf/pgf.h
@@ -99,7 +99,11 @@ pgf_linearize(PgfConcr* concr, PgfExpr expr, GuOut* out, GuExn* err);
 bool
 pgf_parseval(PgfConcr* concr, PgfExpr expr, PgfCId cat, 
              double *precision, double *recall, double *exact);
-                    
+
+PgfExpr
+pgf_compute(PgfPGF* pgf, PgfExpr expr, GuExn* err, 
+            GuPool* pool, GuPool* out_pool);
+
 PgfExprEnum*
 pgf_generate_all(PgfPGF* pgf, PgfCId cat, GuPool* pool);
 
diff --git a/src/runtime/python/pypgf.c b/src/runtime/python/pypgf.c
index ddc1abe12..d31c3e77e 100644
--- a/src/runtime/python/pypgf.c
+++ b/src/runtime/python/pypgf.c
@@ -2301,8 +2301,30 @@ PGF_compute(PGFObject* self, PyObject *args)
     if (!PyArg_ParseTuple(args, "O!", &pgf_ExprType, &py_expr))
 		return NULL;
 
-	Py_INCREF(py_expr);
-	return py_expr;
+	ExprObject* py_expr_res = (ExprObject*) pgf_ExprType.tp_alloc(&pgf_ExprType, 0);
+	if (py_expr_res == NULL)
+		return NULL;
+
+	GuPool* tmp_pool = gu_new_pool();
+	GuExn* err = gu_new_exn(NULL, gu_kind(type), tmp_pool);
+
+	py_expr_res->pool = gu_new_pool();
+	py_expr_res->expr = pgf_compute(self->pgf, py_expr->expr, err, 
+	                                tmp_pool, py_expr_res->pool);
+	py_expr_res->master = (PyObject*) self;
+	Py_INCREF(py_expr_res->master);
+
+	if (!gu_ok(err)) {
+		GuString msg = (GuString) gu_exn_caught_data(err);
+		PyErr_SetString(PGFError, msg);
+
+		Py_DECREF(py_expr_res);
+		gu_pool_free(tmp_pool);
+		return NULL;
+	}
+
+	gu_pool_free(tmp_pool);
+	return py_expr_res;
 }
 
 static ExprObject*