Generic string interpolation (#888)

This implements a general string interpolation package that we intend to
use in the configuration system to allow arbitray interpolation in YAML
values based on environment variables.

This supports variables in the form `${foo}` and `${foo:default}`
anywhere in a string.

build/deps.mk

@@ -21,6 +21,7 @@ EXTRA_GO_BIN_DEPS = \
 GLIDE_VERSION := 0.12.3
 THRIFT_VERSION := 1.0.0-dev
 PROTOC_VERSION := 3.2.0
+RAGEL_VERSION := 6.9
 
 GLIDE_OS := $(UNAME_OS)
 THRIFT_OS := $(UNAME_OS)
@@ -51,9 +52,24 @@ THRIFT = $(BIN)/thrift
 PROTOC_LIB = $(LIB)/protoc-$(PROTOC_VERSION)
 PROTOC_ZIP = $(PROTOC_LIB)/protoc.zip
 PROTOC = $(BIN)/protoc
-GEN_BINS = $(THRIFT) $(PROTOC)
+RAGEL_LIB = $(LIB)/ragel-$(RAGEL_VERSION)
+RAGEL_TAR = $(RAGEL_LIB)/ragel.tar.gz
+RAGEL = $(BIN)/ragel
+
+GEN_BINS = $(THRIFT) $(PROTOC) $(RAGEL)
 EXTRA_BINS = $(GLIDE)
 
+$(RAGEL_TAR):
+	@mkdir -p $(RAGEL_LIB)
+	curl -L "https://www.colm.net/files/ragel/ragel-$(RAGEL_VERSION).tar.gz" > $(RAGEL_TAR)
+
+$(RAGEL): $(RAGEL_TAR)
+	@mkdir -p $(BIN)
+	@cd $(RAGEL_LIB); tar xzf $(RAGEL_TAR)
+	@cd $(RAGEL_LIB)/ragel-$(RAGEL_VERSION); ./configure --prefix=$(RAGEL_LIB) --disable-manual
+	@cd $(RAGEL_LIB)/ragel-$(RAGEL_VERSION); make install
+	@cp $(RAGEL_LIB)/bin/ragel $(RAGEL)
+
 $(GLIDE_TAR):
 	@mkdir -p $(GLIDE_LIB)
 	curl -L "https://github.com/Masterminds/glide/releases/download/v$(GLIDE_VERSION)/glide-v$(GLIDE_VERSION)-$(GLIDE_OS)-$(GLIDE_ARCH).tar.gz" > $(GLIDE_TAR)

internal/interpolate/doc.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2017 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+// Package interpolate provides a generic mechanism to interpolate variables
+// into strings.
+//
+// Variables are specified in the form "${foo}". Values for the different
+// variables are supplied by a VariableResolver function provided at
+// render-time. Variable names can also be in the form "${foo:bar}" where
+// everything after the ":" is the default value for that variable if the
+// VariableResolver did not have a value for that variable.
+package interpolate

internal/interpolate/example_test.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2017 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package interpolate
+
+import "fmt"
+
+var (
+	resolver      = mapResolver(map[string]string{"name": "Inigo Montoya"})
+	emptyResolver = mapResolver(map[string]string{})
+)
+
+func Example() {
+	s, err := Parse("My name is ${name}")
+	if err != nil {
+		panic(err)
+	}
+
+	out, err := s.Render(resolver)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println(out)
+	// Output: My name is Inigo Montoya
+}
+
+func Example_default() {
+	s, err := Parse("My name is ${name:what}")
+	if err != nil {
+		panic(err)
+	}
+
+	out, err := s.Render(emptyResolver)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println(out)
+	// Output: My name is what
+}