Add a compile time string interpolator (#126)

* Add a compile time string interpolator

* Add a test for the string interpolator

core/shared/src/main/scala/cron4s/Interpolator.scala

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2017 Antonio Alonso Dominguez
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cron4s
+
+import cats.implicits._
+
+import contextual._
+
+import cron4s.expr.CronExpr
+
+object CronInterpolator extends Verifier[CronExpr] {
+  def check(input: String) = Cron(input).leftMap {
+    case parseErr: ParseFailed => parseErr.position -> parseErr.getMessage
+    case other: Error          => 0 -> other.getMessage
+  }
+}

core/shared/src/main/scala/cron4s/package.scala

@@ -17,4 +17,12 @@
 import cron4s.syntax.AllSyntax
 import cron4s.expr.NodeConversions
 
-package object cron4s extends AllSyntax with NodeConversions
+import contextual._
+
+package object cron4s extends AllSyntax with NodeConversions {
+
+  implicit class Cron4sStringContext(sc: StringContext) {
+    val cron = Prefix(CronInterpolator, sc)
+  }
+
+}

docs/src/main/tut/docs/index.md

@@ -78,6 +78,28 @@ unless you are aware of the consequences (well, it also comes handy during a REP
 Cron.unsafeParse("10-65 * * * * *")
 ```
 
+#### Compile time expressions
+
+Starting at `cron4s` 0.5.0, there is support for compile-time verified cron expressions, which gives an additional
+guarantee that our expressions are well formed without the need for a runtime check. Compile-time cron expressions
+are basically a string interpolated and prefixed with the `cron` word:
+
+```tut
+val compileTimeChecked = cron"10-35 2,4,6 * ? * *"
+```
+
+As you can see, the return time is not wrapped in an `Either`, the expressions has been verified at compile time and
+therefore there is no need to handle an error case. If we input an invalid cron expression, our code won't compile:
+
+```tut:fail
+val compileFails = cron"10-65 * * * * *"
+```
+
+Now compare that error to the previous one returned by `Cron.unsafeParse`, which was a runtime check. As you can see,
+the compile time error is more helpful but it has its limitations, being the most important one the fact that the
+expression text needs to be resolvable at compile time, if for some reason you are reading your expression as an
+input into your program, you should still fallback to the previous methods.
+
 #### Validation errors
 
 The CRON expression will be validated right after parsing. Any error found during this stage will be returned

docs/src/main/tut/docs/whats_new.md

@@ -11,6 +11,7 @@ New features:
 
  * [#125](https://github.com/alonsodomin/cron4s/pull/125): Add extension module for [decline](http://ben.kirw.in/decline/).
    Find documentation about it in the [extensions](https://alonsodomin.github.io/cron4s/extensions.html) docs.
+ * [#126](https://github.com/alonsodomin/cron4s/pull/126): Add a compile-time string interpolator.
 
 Version upgrades:
 

project/Dependencies.scala

@@ -23,6 +23,8 @@ object Dependencies {
 
     val momentjs      = "0.8.1"
     val scalaJavaTime = "2.0.0-RC1"
+
+    val contextual = "1.1.0"
   }
 
   val macroParadise = compilerPlugin("org.scalamacros" % "paradise"       % "2.1.1" cross CrossVersion.full)
@@ -31,9 +33,10 @@ object Dependencies {
 
   lazy val core = Def.settings {
     libraryDependencies ++= compilerPlugins ++ Seq(
-      "com.chuusai"   %%% "shapeless" % version.shapeless,
-      "org.typelevel" %%% "cats-core" % version.cats,
-      "com.lihaoyi"   %%% "fastparse" % version.fastparse
+      "com.chuusai"    %%% "shapeless" % version.shapeless,
+      "org.typelevel"  %%% "cats-core" % version.cats,
+      "com.lihaoyi"    %%% "fastparse" % version.fastparse,
+      "com.propensive" %%% "contextual" % version.contextual
     )
   }
 

tests/shared/src/test/scala/cron4s/CronSpec.scala

@@ -90,4 +90,10 @@ class CronSpec extends FlatSpec with Matchers {
     Cron.unsafeParse(exprStr) shouldBe ValidExpr
   }
 
+  it should "compile a valid expression" in {
+    val expr = cron"17-30,5 * 12 * * ?"
+
+    expr shouldBe ValidExpr
+  }
+
 }