antonyharfield · April 29, 2019 17:51 · Apr 29, 2019
diff --git a/RailwayOP-CompleteExample.kt b/RailwayOP-CompleteExample.kt
@@ -0,0 +1,77 @@
+// Result is a superpowered enum that can be Success or Failure
+// and the basis for a railway junction
+sealed class Result<T>
+data class Success<T>(val value: T): Result<T>()
+data class Failure<T>(val errorMessage: String): Result<T>()
+
+// Composition: apply a function f to Success results
+infix fun <T,U> Result<T>.then(f: (T) -> Result<U>) = 
+    when (this) {
+        is Success -> f(this.value)
+        is Failure -> Failure(this.errorMessage)
+    }
+
+// Pipe input: the beginning of a railway
+infix fun <T,U> T.to(f: (T) -> Result<U>) = Success(this) then f
+
+// Handle error output: the end of a railway
+infix fun <T> Result<T>.otherwise(f: (String) -> Unit) =
+    if (this is Failure) f(this.errorMessage) else Unit
+
+
+// An example email sending module that reads input, parses, validates, and sends 
+fun main(args: Array<String>) {
+    input() to
+        ::parse then
+        ::validate then
+        ::send otherwise 
+        ::error
+}
+
+data class Email(
+    val to: String,
+    val subject: String,
+    val body: String
+)
+
+// Read in lines of input from the stdin
+fun readLines(prompts: List<String>): List<String> =
+    prompts.map {
+        print("${it}: ")
+        readLine() ?: ""
+    }
+fun input() = readLines(listOf("To", "Subject", "Body"))
+
+// Parse the lines of input to an Email object
+fun parse(inputs: List<String>): Result<Email> = 
+    if (inputs.size == 3)
+        Success(Email(to = inputs[0], subject = inputs[1], body = inputs[2]))
+    else
+        Failure("Unexpected end of input")
+
+// Validate the email address
+fun validAddress(email: Email): Result<Email> = 
+    if (email.to.contains("@"))
+        Success(email)
+    else
+        Failure("Invalid email address")
+
+// Validate the subject and body are not blank
+fun notBlank(email: Email): Result<Email> = 
+    if (email.subject != "" && email.body != "")
+        Success(email)
+    else
+        Failure("Subject and body must not be blank")
+
+// Composition of validation functions
+fun validate(email: Email) = validAddress(email) then ::notBlank
+
+// Send the email (typically this would have an unhappy path too)
+fun send(email: Email): Result<Unit> {
+    println("Sent to ${email.to}. Whoosh!")
+    return Success(Unit)
+}
+
+// The error handler
+fun error(message: String) =
+    println("Something went wrong: ${message}")
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