diff --git a/examples/casestudies/lexer.md b/examples/casestudies/lexer.md
index dd1bdf48e7..57e192822d 100644
--- a/examples/casestudies/lexer.md
+++ b/examples/casestudies/lexer.md
@@ -74,7 +74,8 @@ def example1() = {
 ## Handling the Lexer Effect with a given List
 A dummy lexer reading lexemes from a given list can be implemented as a handler for the `Lexer` effect. The definition uses the effect `LexerError` to signal the end of the input stream:
 ```
-effect LexerError(msg: String, pos: Position): Nothing
+effect LexerError(msg: String, pos: Position): Unit
+def absurd[A](unit: Unit): A = panic("should not happen")
 def dummyPosition() = Position(0, 0, 0)
 
 def lexerFromList[R](l: List[Token]) { program: => R / Lexer }: R / LexerError = {
@@ -85,7 +86,7 @@ def lexerFromList[R](l: List[Token]) { program: => R / Lexer }: R / LexerError =
       case Cons(tok, _) => resume(Some(tok))
     }
     def next() = in match {
-      case Nil() => do LexerError("Unexpected end of input", dummyPosition())
+      case Nil() => do LexerError("Unexpected end of input", dummyPosition()).absurd
       case Cons(tok, _) => resume(tok)
     }
   }
@@ -192,10 +193,10 @@ the input, or not.
     def peek() = resume(tryMatchAll(tokenDesriptors()))
     def next() =
       if (eos())
-        do LexerError("Unexpected EOS", position())
+        do LexerError("Unexpected EOS", position()).absurd
       else {
         val tok = tryMatchAll(tokenDesriptors()).getOrElse {
-          do LexerError("Cannot tokenize input", position())
+          do LexerError("Cannot tokenize input", position()).absurd
         }
         consume(tok.text)
         resume(tok)
diff --git a/examples/casestudies/parser.md b/examples/casestudies/parser.md
index 5c065c7de4..1afe285f2a 100644
--- a/examples/casestudies/parser.md
+++ b/examples/casestudies/parser.md
@@ -25,7 +25,7 @@ Parsers can be expressed by using the lexer effect and process the token stream.
 ```
 effect Nondet {
   def alt(): Boolean
-  def fail[A](msg: String): A
+  def fail(msg: String): Unit
 }
 
 effect Parser = { Nondet, Lexer }
@@ -40,7 +40,7 @@ input stream and fails, if it does not match.
 def accept { p: Token => Boolean } : Token / Parser = {
   val got = do next();
   if (p(got)) got
-  else do fail("Unexpected token " ++ show(got))
+  else do fail("Unexpected token " ++ show(got)).absurd
 }
 ```
 
@@ -53,12 +53,12 @@ def number() = accept(Number()).text
 def punct(p: String) = {
   val tok = accept(Punct())
   if (tok.text == p) ()
-  else do fail("Expected " ++ p ++ " but got " ++ tok.text)
+  else do fail("Expected " ++ p ++ " but got " ++ tok.text).absurd
 }
 def kw(exp: String): Unit / Parser = {
   val got = ident();
   if (got == exp) ()
-  else do fail("Expected keyword " ++ exp ++ " but got " ++ got)
+  else do fail("Expected keyword " ++ exp ++ " but got " ++ got).absurd
 }
 ```
 Using the effect for non-deterministic choice `alt`, we can model alternatives, optional matches and various repetitions:
@@ -100,7 +100,7 @@ Let us start by defining the parser for numeric literals.
 def parseNum(): Tree / Parser = {
   val numText = number()
   val num = toInt(numText).getOrElse {
-    do fail("Expected number, but cannot convert input to integer: " ++ numText)
+    do fail("Expected number, but cannot convert input to integer: " ++ numText).absurd
   }
   Lit(num)
 }
@@ -122,17 +122,8 @@ semantics of effects.
 
 Similarly, we can write parsers for let bindings, by sequentially composing
 our existing parsers:
-```
-def parseLet(): Tree / Parser = {
-  kw("let");
-  val name = ident();
-  punct("=");
-  val binding = parseExpr();
-  kw("in");
-  val body = parseExpr();
-  Let(name, binding, body)
-}
-```
+
+
 Again, note how naturally the result can be composed from the individual results, much like
 manually writing a recursive descent parser. Compared to handcrafted parsers, the imperative
 parser combinators presented here offer a similar flexibility. At the same time, the semantics
@@ -140,23 +131,35 @@ of `alt` and `fail` is still left open, offering flexibility in the implementati
 
 We proceed to implement the remaining parsers for our expression language:
 ```
-def parseGroup() = or { parseAtom() } {
-  punct("(");
-  val res = parseExpr();
-  punct(")");
-  res
-}
+def parseExpr(): Tree / Parser = {
+
+  def parseLet(): Tree / {} = {
+    kw("let");
+    val name = ident();
+    punct("=");
+    val binding = parseExpr();
+    kw("in");
+    val body = parseExpr();
+    Let(name, binding, body)
+  }
 
-def parseApp(): Tree / Parser = {
-  val funName = ident();
-  punct("(");
-  val arg = parseExpr();
-  punct(")");
-  App(funName, arg)
-}
+  def parseGroup(): Tree / {} = or { parseAtom() } {
+    punct("(");
+    val res = parseExpr();
+    punct(")");
+    res
+  }
+
+  def parseApp(): Tree / {} = {
+    val funName = ident();
+    punct("(");
+    val arg = parseExpr();
+    punct(")");
+    App(funName, arg)
+  }
 
-def parseExpr(): Tree / Parser =
   or { parseLet() } { or { parseApp() } { parseGroup() } }
+}
 ```
 
 ## Example: Combining Parsers and Local Mutable State
@@ -219,8 +222,8 @@ def parse[R](input: String) { p: => R / Parser }: ParseResult[R] = try {
     case Failure(msg) => resume(false)
     case Success(res) => Success(res)
   }
-  def fail[A](msg) = Failure(msg)
-} with LexerError[A] { (msg, pos) =>
+  def fail(msg) = Failure(msg)
+} with LexerError { (msg, pos) =>
   Failure(msg)
 }
 ```
@@ -240,11 +243,11 @@ def println(p: ParseResult[Int]): Unit = println(showPR(p){ x => show(x) })
 def println(p: ParseResult[Tree]): Unit = println(showPR(p){ x => show(x) })
 def main() = {
   println(parse("42") { parseCalls() })
-  println(parse("foo(1)") { parseCalls() })
-  println(parse("foo(1, 2)") { parseCalls() })
-  println(parse("foo(1, 2, 3, 4)") { parseCalls() })
-  println(parse("foo(1, 2, bar(4, 5))") { parseCalls() })
-  println(parse("foo(1, 2,\nbar(4, 5))") { parseCalls() })
+//  println(parse("foo(1)") { parseCalls() })
+//  println(parse("foo(1, 2)") { parseCalls() })
+//  println(parse("foo(1, 2, 3, 4)") { parseCalls() })
+//  println(parse("foo(1, 2, bar(4, 5))") { parseCalls() })
+//  println(parse("foo(1, 2,\nbar(4, 5))") { parseCalls() })
 
 //   println(parse("}42") { parseExpr() })
 //   println(parse("42") { parseExpr() })
diff --git a/examples/casestudies/prettyprinter.md b/examples/casestudies/prettyprinter.md
index 58f794f8bc..31db10eb14 100644
--- a/examples/casestudies/prettyprinter.md
+++ b/examples/casestudies/prettyprinter.md
@@ -19,7 +19,6 @@ Furthermore, the library presented here is neither linear
 ```
 module examples/casestudies/prettyprinter
 
-import examples/casestudies/parser // just needed for the example (Tree)
 import immutable/option
 import immutable/list
 import text/string
diff --git a/libraries/ml/effekt.effekt b/libraries/ml/effekt.effekt
index 087fb6f522..3e03956d42 100644
--- a/libraries/ml/effekt.effekt
+++ b/libraries/ml/effekt.effekt
@@ -189,7 +189,7 @@ record Tuple6[A, B, C, D, E, F](first: A, second: B, third: C, fourth: D, fifth:
 extern io def panic[R](msg: String): R =
   "raise Fail msg"
 
-effect Exception[E] {
+interface Exception[E] {
   def raise(exception: E, msg: String): Nothing
 }
 record RuntimeError()
@@ -211,7 +211,7 @@ def ignoring[E] { prog: => Unit / Exception[E] }: Unit =
 
 // Control Flow
 // ============
-effect Control {
+interface Control {
   def break(): Unit
   def continue(): Unit
 }
diff --git a/libraries/ml/text/string.effekt b/libraries/ml/text/string.effekt
index 0b20109656..fc8d22a339 100644
--- a/libraries/ml/text/string.effekt
+++ b/libraries/ml/text/string.effekt
@@ -10,8 +10,8 @@ import internal/option
 
 def charAt(str: String, index: Int): Option[String] =
     if (index < 0 || length(str) <= index)
-        Some(unsafeCharAt(str, index))
-    else None()
+        None()
+    else Some(unsafeCharAt(str, index))
 
 def takeWhile(str: String){ f: String => Boolean }: String = {
     def rec(i: Int, acc: String): String = {
@@ -47,9 +47,11 @@ def substring(str: String, from: Int): String =
         str
     else unsafeSubstring(str, from)
 def substring(str: String, from: Int, endp: Int): String =
-    if (from < 0 || length(str) <= from || length(str) <= endp)
+    if (from < 0 || length(str) <= from)
         str
-    else unsafeSubstring(str, from, endp)
+    else if (length(str) <= endp) 
+        unsafeSubstring(str, from) 
+    else unsafeSubstring(str, from, endp-from)
 
 extern pure def unsafeSubstring(str: String, from: Int): String =
     "String.extract (str, from, NONE)"