envexp: add placeholder for Expander

Signed-off-by: Alejandro Mery <[email protected]>

envexp/expand.go

@@ -0,0 +1,43 @@
+// Package envexp provides environment variable expansion using sh syntax
+package envexp
+
+import (
+	"os"
+)
+
+// A VarGetter is a function that for a given paramenter
+// returns a string representing its value
+type VarGetter func(key string) string
+
+// A Expander is an object that you can use to Expand strings
+type Expander interface {
+	Expand(s string) string
+}
+
+type expander struct {
+	get VarGetter
+}
+
+// NewExpander allocates a new Expander using a
+// given VarGetter, or os.Getenv if none is given
+func NewExpander(f VarGetter) Expander {
+	if f == nil {
+		f = os.Getenv
+	}
+
+	return &expander{
+		get: f,
+	}
+}
+
+// Get resolves a variable name as the given Expander
+// would do
+func (exp *expander) Get(key string) string {
+	return exp.get(key)
+}
+
+// Expand expands a string using the VarGetter
+// defined for the given Expander
+func (exp *expander) Expand(s string) string {
+	return s
+}