Implement all builtins required to get fib working

Getting `recur` to work required adding an Operation argument to a bunch
of places in a really hacky way. I don't like it at all, but I'm also
kind of out of mental energy to figure it out properly. The fibonacci
demo in the README _works_, at least, though I don't think it's actually
tail recursive.

README.md

@@ -4,17 +4,23 @@ Fibonacci function in ginger:
 
 ```
 fib = {
+
     decr = { out = add < (in; -1;); };
 
     out = {
-        n = 0 < in;
-        a = 1 < in;
-        b = 2 < in;
 
-        out < if < (
+        n = tupEl < (in; 0;);
+        a = tupEl < (in; 1;);
+        b = tupEl < (in; 2;);
+
+        out = if < (
             zero? < n;
             a;
-            recur < (decr < n; b; add < (a;b;); );
+            recur < (
+                decr < n;
+                b;
+                add < (a;b;);
+            );
         );
 
     } < (in; 0; 1;);

vm/op.go

@@ -46,15 +46,18 @@ func evalThunks(args []Thunk) Thunk {
 
 
 // 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.
+// having been evaluated yet) and return a Thunk which will perform some
+// internal processing on those arguments and return a resultant Value.
+//
+// The Operation passed into Perform is the Operation which is calling the
+// Perform. It may be nil.
 type Operation interface {
-	Perform([]Thunk) (Thunk, error)
+	Perform([]Thunk, Operation) (Thunk, error)
 }
 
 func preEvalValOp(fn func(Value) (Value, error)) Operation {
 
-	return OperationFunc(func(args []Thunk) (Thunk, error) {
+	return OperationFunc(func(args []Thunk, _ Operation) (Thunk, error) {
 
 		return func() (Value, error) {
 
@@ -89,18 +92,19 @@ func OperationFromGraph(g *gg.Graph, scope Scope) Operation {
 	}
 }
 
-func (g *graphOp) Perform(args []Thunk) (Thunk, error) {
+func (g *graphOp) Perform(args []Thunk, _ Operation) (Thunk, error) {
 	return ScopeFromGraph(
 		g.Graph,
 		evalThunks(args),
 		g.scope,
+		g,
 	).Evaluate(outVal)
 }
 
 // OperationFunc is a function which implements the Operation interface.
-type OperationFunc func([]Thunk) (Thunk, error)
+type OperationFunc func([]Thunk, Operation) (Thunk, error)
 
 // Perform calls the underlying OperationFunc directly.
-func (f OperationFunc) Perform(args []Thunk) (Thunk, error) {
-	return f(args)
+func (f OperationFunc) Perform(args []Thunk, op Operation) (Thunk, error) {
+	return f(args, op)
 }

vm/scope.go

@@ -24,7 +24,9 @@ type Scope interface {
 // 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) {
+//
+// The Operation is the Operation which is evaluating the edge, if any.
+func EvaluateEdge(edge *gg.OpenEdge, scope Scope, op Operation) (Value, error) {
 
 	thunk, err := graph.MapReduce[gg.Value, gg.Value, Thunk](
 		edge,
@@ -73,7 +75,7 @@ func EvaluateEdge(edge *gg.OpenEdge, scope Scope) (Value, error) {
 				return nil, fmt.Errorf("edge value must be an operation")
 			}
 
-			return edgeVal.Operation.Perform(args)
+			return edgeVal.Operation.Perform(args, op)
 		},
 	)
 
@@ -115,6 +117,7 @@ type graphScope struct {
 	*gg.Graph
 	in Thunk
 	parent Scope
+	op Operation
 }
 
 // ScopeFromGraph returns a Scope which will use the given Graph for evaluation.
@@ -133,11 +136,12 @@ type graphScope struct {
 //
 // NewScope will return the parent scope, if one is given, or an empty ScopeMap
 // if not.
-func ScopeFromGraph(g *gg.Graph, in Thunk, parent Scope) Scope {
+func ScopeFromGraph(g *gg.Graph, in Thunk, parent Scope, op Operation) Scope {
 	return &graphScope{
 		Graph:  g,
 		in: in,
 		parent: parent,
+		op: op,
 	}
 }
 
@@ -165,7 +169,9 @@ func (g *graphScope) Evaluate(nameVal Value) (Thunk, error) {
 		)
 	}
 
-	return func() (Value, error) { return EvaluateEdge(edgesIn[0], g) }, nil
+	return func() (Value, error) {
+		return EvaluateEdge(edgesIn[0], g, g.op)
+	}, nil
 }
 
 func (g *graphScope) NewScope() Scope {

vm/scope_global.go

@@ -12,10 +12,6 @@ var GlobalScope = ScopeMap{
 
 	"add": Value{Operation: preEvalValOp(func(val Value) (Value, error) {
 
-		if len(val.Tuple) == 0 {
-			return Value{}, fmt.Errorf("add requires a non-zero tuple of numbers as an argument")
-		}
-
 		var sum int64
 
 		for _, tupVal := range val.Tuple {
@@ -30,4 +26,53 @@ var GlobalScope = ScopeMap{
 		return Value{Value: gg.Value{Number: &sum}}, nil
 
 	})},
+
+	"tupEl": Value{Operation: preEvalValOp(func(val Value) (Value, error) {
+
+		tup, i := val.Tuple[0], val.Tuple[1]
+
+		return tup.Tuple[int(*i.Number)], nil
+
+	})},
+
+	"isZero": Value{Operation: preEvalValOp(func(val Value) (Value, error) {
+
+		if *val.Number == 0 {
+			one := int64(1)
+			return Value{Value: gg.Value{Number: &one}}, nil
+		}
+
+		zero := int64(0)
+		return Value{Value: gg.Value{Number: &zero}}, nil
+
+	})},
+
+	"if": Value{Operation: OperationFunc(func(args []Thunk, _ Operation) (Thunk, error) {
+
+		b := args[0]
+		onTrue := args[1]
+		onFalse := args[2]
+
+		return func() (Value, error) {
+
+			bVal, err := b()
+
+			if err != nil {
+				return ZeroValue, err
+			}
+
+			if *bVal.Number == 0 {
+				return onFalse()
+			}
+
+			return onTrue()
+
+		}, nil
+
+	})},
+
+	"recur": Value{Operation: OperationFunc(func(args []Thunk, op Operation) (Thunk, error) {
+		return op.Perform(args, op)
+	})},
+
 }

vm/vm.go

@@ -115,9 +115,10 @@ func EvaluateSource(opSrc io.Reader, input gg.Value, scope Scope) (Value, error)
 
 	op := OperationFromGraph(g, scope.NewScope())
 
-	thunk, err := op.Perform([]Thunk{
-		func() (Value, error) { return Value{Value: input}, nil },
-	})
+	thunk, err := op.Perform(
+		[]Thunk{valThunk(Value{Value: input})},
+		nil,
+	)
 
 	if err != nil {
 		return ZeroValue, err