Refactor vm to use MapReduce and Thunks

The new code is much simpler, and is able to handle more cases than
before, such as the `if` operation.

vm/op.go

@@ -2,70 +2,73 @@ package vm
 
 import (
 	"github.com/mediocregopher/ginger/gg"
-	"github.com/mediocregopher/ginger/graph"
 )
 
 var (
-	inVal  = nameVal("in")
 	outVal = nameVal("out")
 )
 
-// Operation is an entity which can accept a single argument (the OpenEdge),
-// perform some internal processing on that argument, and return a resultant
-// Value.
-//
-// The Scope passed into Perform can be used to Evaluate the OpenEdge, as
-// needed.
-type Operation interface {
-	Perform(*gg.OpenEdge, Scope) (Value, error)
+// Thunk is returned from the performance of an Operation. When called it will
+// return the result of that Operation having been called with the particular
+// arguments which were passed in.
+type Thunk func() (Value, error)
+
+func valThunk(val Value) Thunk {
+	return func() (Value, error) { return val, nil }
 }
 
-func preEvalValOp(fn func(Value) (Value, error)) Operation {
+// evalThunks is used to coalesce the results of multiple Thunks into a single
+// Thunk which will return a tuple Value. As a special case, if only one Thunk
+// is given then it is returned directly (1-tuple is equivalent to its only
+// element).
+func evalThunks(args []Thunk) Thunk {
 
-	return OperationFunc(func(edge *gg.OpenEdge, scope Scope) (Value, error) {
+	if len(args) == 1 {
+		return args[0]
+	}
+
+	return func() (Value, error) {
 
-		edgeVal, err := EvaluateEdge(edge, scope)
+		var (
+			err error
+			tupVals = make([]Value, len(args))
+		)
 
-		if err != nil {
-			return Value{}, err
+		for i := range args {
+			if tupVals[i], err = args[i](); err != nil {
+				return ZeroValue, err
+			}
 		}
 
-		return fn(edgeVal)
-	})
+		return Value{Tuple: tupVals}, nil
+	}
 }
 
-// NOTE this is a giant hack to get around the fact that we're not yet
-// using a genericized Graph implementation, so when we do AddValueIn
-// on a gg.Graph we can't use a Tuple value (because gg has no Tuple
-// value), we have to use a Tuple vertex instead.
-//
-// This also doesn't yet support passing an operation as a value to another
-// operation.
-func preEvalEdgeOp(fn func(*gg.OpenEdge) (Value, error)) Operation {
-
-	return preEvalValOp(func(val Value) (Value, error) {
 
-		var edge *gg.OpenEdge
+// Operation is an entity which can accept one or more arguments (each not
+// having been evaluated yet) and return a Thunk which will perform some internal processing on those
+// arguments and return a resultant Value.
+type Operation interface {
+	Perform([]Thunk) (Thunk, error)
+}
 
-		if len(val.Tuple) > 0 {
+func preEvalValOp(fn func(Value) (Value, error)) Operation {
 
-			tupEdges := make([]*gg.OpenEdge, len(val.Tuple))
+	return OperationFunc(func(args []Thunk) (Thunk, error) {
 
-			for i := range val.Tuple {
-				tupEdges[i] = graph.ValueOut[gg.Value](gg.ZeroValue, val.Tuple[i].Value)
-			}
+		return func() (Value, error) {
 
-			edge = graph.TupleOut[gg.Value](gg.ZeroValue, tupEdges...)
+			val, err := evalThunks(args)()
 
-		} else {
+			if err != nil {
+				return ZeroValue, err
+			}
 
-			edge = graph.ValueOut[gg.Value](gg.ZeroValue, val.Value)
+			return fn(val)
 
-		}
+		}, nil
 
-		return fn(edge)
 	})
-
 }
 
 type graphOp struct {
@@ -76,10 +79,9 @@ type graphOp struct {
 // OperationFromGraph wraps the given Graph such that it can be used as an
 // operation.
 //
-// When Perform is called the passed in OpenEdge is set to the "in" name value
-// of the given Graph, then that resultant graph and the given parent Scope are
-// used to construct a new Scope. The "out" name value is Evaluated on that
-// Scope to obtain a resultant Value.
+// The Thunk returned by Perform will evaluate the passed in Thunks, and set
+// them to the "in" name value of the given Graph. The "out" name value is
+// Evaluated using the given Scope to obtain a resultant Value.
 func OperationFromGraph(g *gg.Graph, scope Scope) Operation {
 	return &graphOp{
 		Graph: g,
@@ -87,25 +89,18 @@ func OperationFromGraph(g *gg.Graph, scope Scope) Operation {
 	}
 }
 
-func (g *graphOp) Perform(edge *gg.OpenEdge, scope Scope) (Value, error) {
-
-	return preEvalEdgeOp(func(edge *gg.OpenEdge) (Value, error) {
-
-		scope = ScopeFromGraph(
-			g.Graph.AddValueIn(inVal.Value, edge),
-			g.scope,
-		)
-
-		return scope.Evaluate(outVal)
-
-	}).Perform(edge, scope)
-
+func (g *graphOp) Perform(args []Thunk) (Thunk, error) {
+	return ScopeFromGraph(
+		g.Graph,
+		evalThunks(args),
+		g.scope,
+	).Evaluate(outVal)
 }
 
 // OperationFunc is a function which implements the Operation interface.
-type OperationFunc func(*gg.OpenEdge, Scope) (Value, error)
+type OperationFunc func([]Thunk) (Thunk, error)
 
 // Perform calls the underlying OperationFunc directly.
-func (f OperationFunc) Perform(edge *gg.OpenEdge, scope Scope) (Value, error) {
-	return f(edge, scope)
+func (f OperationFunc) Perform(args []Thunk) (Thunk, error) {
+	return f(args)
 }

vm/scope.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 
 	"github.com/mediocregopher/ginger/gg"
+	"github.com/mediocregopher/ginger/graph"
 )
 
 // Scope encapsulates a set of names and the values they indicate, or the means
@@ -11,86 +12,76 @@ import (
 // its value.
 type Scope interface {
 
-	// Evaluate accepts a name Value and returns the real Value which that name
-	// points to.
-	Evaluate(Value) (Value, error)
+	// Evaluate accepts a name Value and returns a Thunk which will return the
+	// real Value which that name points to.
+	Evaluate(Value) (Thunk, error)
 
 	// NewScope returns a new Scope which sub-operations within this Scope
 	// should use for themselves.
 	NewScope() Scope
 }
 
-// edgeToValue ignores the edgeValue, it only evaluates the edge's vertex as a
-// Value.
-func edgeToValue(edge *gg.OpenEdge, scope Scope) (Value, error) {
-
-	if ggVal, ok := edge.FromValue(); ok {
-
-		val := Value{Value: ggVal}
-
-		if val.Name != nil {
-			return scope.Evaluate(val)
-		}
-
-		return val, nil
-	}
-
-	var tupVal Value
-
-	tup, _ := edge.FromTuple()
+// EvaluateEdge will use the given Scope to evaluate the edge's ultimate Value,
+// after passing all leaf vertices up the tree through all Operations found on
+// edge values.
+func EvaluateEdge(edge *gg.OpenEdge, scope Scope) (Value, error) {
 
-	for _, tupEdge := range tup {
+	thunk, err := graph.MapReduce[gg.Value, gg.Value, Thunk](
+		edge,
+		func(ggVal gg.Value) (Thunk, error) {
 
-		val, err := EvaluateEdge(tupEdge, scope)
+			val := Value{Value: ggVal}
 
-		if err != nil {
-			return Value{}, err
-		}
+			if val.Name != nil {
+				return scope.Evaluate(val)
+			}
 
-		tupVal.Tuple = append(tupVal.Tuple, val)
-	}
+			return valThunk(val), nil
 
-	if len(tupVal.Tuple) == 1 {
-		return tupVal.Tuple[0], nil
-	}
+		},
+		func(ggEdgeVal gg.Value, args []Thunk) (Thunk, error) {
 
-	return tupVal, nil
-}
+			if ggEdgeVal.IsZero() {
+				return evalThunks(args), nil
+			}
 
-// EvaluateEdge will use the given Scope to evaluate the edge's ultimate Value,
-// after passing all leaf vertices up the tree through all Operations found on
-// edge values.
-func EvaluateEdge(edge *gg.OpenEdge, scope Scope) (Value, error) {
+			edgeVal := Value{Value: ggEdgeVal}
 
-	edgeVal := Value{Value: edge.EdgeValue()}
+			if edgeVal.Name != nil {
 
-	if edgeVal.IsZero() {
-		return edgeToValue(edge, scope)
-	}
+				nameThunk, err := scope.Evaluate(edgeVal)
 
-	edge = edge.WithEdgeValue(gg.ZeroValue)
+				if err != nil {
+					return nil, err
 
-	if edgeVal.Name != nil {
+				} else if edgeVal, err = nameThunk(); err != nil {
+					return nil, err
+				}
+			}
 
-		var err error
+			if edgeVal.Graph != nil {
 
-		if edgeVal, err = scope.Evaluate(edgeVal); err != nil {
-			return Value{}, err
-		}
-	}
+				edgeVal = Value{
+					Operation: OperationFromGraph(
+						edgeVal.Graph,
+						scope.NewScope(),
+					),
+				}
+			}
 
-	if edgeVal.Graph != nil {
+			if edgeVal.Operation == nil {
+				return nil, fmt.Errorf("edge value must be an operation")
+			}
 
-		edgeVal = Value{
-			Operation: OperationFromGraph(edgeVal.Graph, scope.NewScope()),
-		}
-	}
+			return edgeVal.Operation.Perform(args)
+		},
+	)
 
-	if edgeVal.Operation == nil {
-		return Value{}, fmt.Errorf("edge value must be an operation")
+	if err != nil {
+		return ZeroValue, err
 	}
 
-	return edgeVal.Operation.Perform(edge, scope)
+	return thunk()
 }
 
 // ScopeMap implements the Scope interface.
@@ -100,19 +91,19 @@ var _ Scope = ScopeMap{}
 
 // Evaluate uses the given name Value as a key into the ScopeMap map, and
 // returns the Value held there for the key, if any.
-func (m ScopeMap) Evaluate(nameVal Value) (Value, error) {
+func (m ScopeMap) Evaluate(nameVal Value) (Thunk, error) {
 
 	if nameVal.Name == nil {
-		return Value{}, fmt.Errorf("value %v is not a name value", nameVal)
+		return nil, fmt.Errorf("value %v is not a name value", nameVal)
 	}
 
 	val, ok := m[*nameVal.Name]
 
 	if !ok {
-		return Value{}, fmt.Errorf("%q not defined", *nameVal.Name)
+		return nil, fmt.Errorf("%q not defined", *nameVal.Name)
 	}
 
-	return val, nil
+	return valThunk(val), nil
 }
 
 // NewScope returns the ScopeMap as-is.
@@ -122,6 +113,7 @@ func (m ScopeMap) NewScope() Scope {
 
 type graphScope struct {
 	*gg.Graph
+	in Thunk
 	parent Scope
 }
 
@@ -132,23 +124,31 @@ type graphScope struct {
 // be followed, with edge values being evaluated to Operations, until a Value
 // can be obtained.
 //
+// As a special case, if the name "in" is evaluated, either directly or as part
+// of an outer evaluation, then the given Thunk is used to evaluate the Value.
+//
 // If a name does not appear in the Graph, then the given parent Scope will be
 // used to evaluate that name. If the parent Scope is nil then an error is
 // returned.
 //
 // NewScope will return the parent scope, if one is given, or an empty ScopeMap
 // if not.
-func ScopeFromGraph(g *gg.Graph, parent Scope) Scope {
+func ScopeFromGraph(g *gg.Graph, in Thunk, parent Scope) Scope {
 	return &graphScope{
 		Graph:  g,
+		in: in,
 		parent: parent,
 	}
 }
 
-func (g *graphScope) Evaluate(nameVal Value) (Value, error) {
+func (g *graphScope) Evaluate(nameVal Value) (Thunk, error) {
 
 	if nameVal.Name == nil {
-		return Value{}, fmt.Errorf("value %v is not a name value", nameVal)
+		return nil, fmt.Errorf("value %v is not a name value", nameVal)
+	}
+
+	if *nameVal.Name == "in" {
+		return g.in, nil
 	}
 
 	edgesIn := g.ValueIns(nameVal.Value)
@@ -159,13 +159,13 @@ func (g *graphScope) Evaluate(nameVal Value) (Value, error) {
 
 	} else if l != 1 {
 
-		return Value{}, fmt.Errorf(
+		return nil, fmt.Errorf(
 			"%q must have exactly one input edge, found %d input edges",
 			*nameVal.Name, l,
 		)
 	}
 
-	return EvaluateEdge(edgesIn[0], g)
+	return func() (Value, error) { return EvaluateEdge(edgesIn[0], g) }, nil
 }
 
 func (g *graphScope) NewScope() Scope {

vm/vm.go

@@ -115,5 +115,13 @@ func EvaluateSource(opSrc io.Reader, input gg.Value, scope Scope) (Value, error)
 
 	op := OperationFromGraph(g, scope.NewScope())
 
-	return op.Perform(graph.ValueOut[gg.Value](gg.ZeroValue, input), scope)
+	thunk, err := op.Perform([]Thunk{
+		func() (Value, error) { return Value{Value: input}, nil },
+	})
+
+	if err != nil {
+		return ZeroValue, err
+	}
+
+	return thunk()
 }