Extract ModuleGetter interface for dynamic imports (#349)

* Extract ModuleGetter interface for dynamic imports

* ModuleGetter: add example in interop doc.

compiler.go

@@ -49,7 +49,7 @@ type Compiler struct {
 	symbolTable     *SymbolTable
 	scopes          []compilationScope
 	scopeIndex      int
-	modules         *ModuleMap
+	modules         ModuleGetter
 	compiledModules map[string]*CompiledFunction
 	allowFileImport bool
 	loops           []*loop
@@ -63,7 +63,7 @@ func NewCompiler(
 	file *parser.SourceFile,
 	symbolTable *SymbolTable,
 	constants []Object,
-	modules *ModuleMap,
+	modules ModuleGetter,
 	trace io.Writer,
 ) *Compiler {
 	mainScope := compilationScope{

docs/interoperability.md

@@ -168,6 +168,32 @@ mods.AddSourceModule("double", []byte(`export func(x) { return x * 2 }`))
 s.SetImports(mods)
 ```
 
+To dynamically load or generate code for imported modules, implement and
+provide a `tengo.ModuleGetter`.
+
+```golang
+type DynamicModules struct {
+  mods tengo.ModuleGetter
+  fallback func (name string) tengo.Importable
+}
+func (dm *DynamicModules) Get(name string) tengo.Importable {
+  if mod := dm.mods.Get(name); mod != nil {
+    return mod
+  }
+  return dm.fallback()
+}
+// ...
+mods := &DynamicModules{
+  mods: stdlib.GetModuleMap("math"),
+  fallback: func(name string) tengo.Importable {
+    src := ... // load or generate src for `name`
+    return &tengo.SourceModule{Src: src}
+  },
+}
+s := tengo.NewScript(`foo := import("foo")`)
+s.SetImports(mods)
+```
+
 ### Script.SetMaxAllocs(n int64)
 
 SetMaxAllocs sets the maximum number of object allocations. Note this is a

modules.go

@@ -6,6 +6,11 @@ type Importable interface {
 	Import(moduleName string) (interface{}, error)
 }
 
+// ModuleGetter enables implementing dynamic module loading.
+type ModuleGetter interface {
+	Get(name string) Importable
+}
+
 // ModuleMap represents a set of named modules. Use NewModuleMap to create a
 // new module map.
 type ModuleMap struct {

script.go

@@ -12,7 +12,7 @@ import (
 // Script can simplify compilation and execution of embedded scripts.
 type Script struct {
 	variables        map[string]*Variable
-	modules          *ModuleMap
+	modules          ModuleGetter
 	input            []byte
 	maxAllocs        int64
 	maxConstObjects  int
@@ -54,7 +54,7 @@ func (s *Script) Remove(name string) bool {
 }
 
 // SetImports sets import modules.
-func (s *Script) SetImports(modules *ModuleMap) {
+func (s *Script) SetImports(modules ModuleGetter) {
 	s.modules = modules
 }