Format and some docs

docs/expressions.md

@@ -86,3 +86,15 @@ Uses truthy logic and applies `and` or `or` to the values.
 
 and: All arguments need to be truthy
 or:  At least one argument needs to be truthy
+
+## Like, Prefix, Suffix
+
+Syntax: `{like val contains}`, `{prefix val startsWith}`, `{suffix val endsWith}`
+
+Truthy check if a value contains a sub-value, starts with, or ends with
+
+## Format
+
+Syntax: `{format "%s" ...}`
+
+Formats a string based on `fmt.Sprintf`

pkg/expressions/keyFunctions.go

@@ -241,6 +241,24 @@ func kfSuffix(args []KeyBuilderStage) KeyBuilderStage {
 	})
 }
 
+// {format str args...}
+// just like fmt.Sprintf
+func kfFormat(args []KeyBuilderStage) KeyBuilderStage {
+	return KeyBuilderStage(func(context KeyBuilderContext) string {
+		if len(args) < 1 {
+			return ErrorArgCount
+		}
+		format := args[0](context)
+
+		printArgs := make([]interface{}, len(args)-1)
+		for idx, stage := range args[1:] {
+			printArgs[idx] = stage(context)
+		}
+
+		return fmt.Sprintf(format, printArgs...)
+	})
+}
+
 var defaultFunctions = map[string]KeyBuilderFunction{
 	"coalesce":  KeyBuilderFunction(kfCoalesce),
 	"bucket":    KeyBuilderFunction(kfBucket),
@@ -272,4 +290,5 @@ var defaultFunctions = map[string]KeyBuilderFunction{
 	"like":   KeyBuilderFunction(kfLike),
 	"prefix": KeyBuilderFunction(kfPrefix),
 	"suffix": KeyBuilderFunction(kfSuffix),
+	"format": KeyBuilderFunction(kfFormat),
 }