Exit function (#487)

* Exit function

Introduce the `exit` function as a means of terminating the program
early. This also requires the introduction of the `@noreturn` decorator
to indicate that code following the invocation of such a function is
not reachable.

* Emit 'hlt' instruction when invoking a Never function

* Ensure match expression blocks that contain noreturn function calls don't do a 'store'

projects/compiler/example.abra

@@ -1,16 +1,2 @@
-func ok(): Result<Int, String> = Ok(123)
-func err(): Result<Int, String> = Err("foo")
-
-func f1(): Result<Int, Int> {
-  var acc = 0
-  while acc < 10 {
-    val x = try err() else break
-    println(x)
-    acc += x
-  }
-
-  Ok(acc)
-}
-
-/// Expect: Result.Ok(value: 124)
-println(f1())
+val x = if true 123 else Process.exit()
+println("x =", x)

projects/compiler/src/compiler.abra

@@ -1429,6 +1429,10 @@ export type Compiler {
           }
         }
 
+        if node.ty.kind == TypeKind.Never {
+          self._currentFn.block.buildHalt()
+        }
+
         Ok(res)
       }
       TypedAstNodeKind.Array(items) => {
@@ -1698,11 +1702,13 @@ export type Compiler {
         for node, idx in ifBlock {
           val res = try self._compileStatement(node)
           if idx == ifBlock.length - 1 {
-            if res |res| {
-              val label = self._currentFn.block.currentLabel
-              phiCases.push((label, res))
-            } else if !ifBlockTerminator {
-              return unreachable("last statement in if-expr block has no value and is not a terminator", node.token.position)
+            if !ifBlockTerminator {
+              if res |res| {
+                val label = self._currentFn.block.currentLabel
+                phiCases.push((label, res))
+              } else {
+                return unreachable("last statement in if-expr block has no value and is not a terminator", node.token.position)
+              }
             }
           }
         }
@@ -1715,11 +1721,13 @@ export type Compiler {
           for node, idx in elseBlock {
             val res = try self._compileStatement(node)
             if idx == elseBlock.length - 1 {
-              if res |res| {
-                val label = self._currentFn.block.currentLabel
-                phiCases.push((label, res))
-              } else if !elseBlockTerminator {
-                return unreachable("last statement in if-expr else block has no value and is not a terminator", node.token.position)
+              if !elseBlockTerminator {
+                if res |res| {
+                  val label = self._currentFn.block.currentLabel
+                  phiCases.push((label, res))
+                } else {
+                  return unreachable("last statement in if-expr else block has no value and is not a terminator", node.token.position)
+                }
               }
             }
           }
@@ -1966,8 +1974,10 @@ export type Compiler {
         val res = try self._compileStatement(node)
         if idx == case.body.length - 1 {
           if resValSlotCtx |(slot, slotTy)| {
-            if res |res| {
-              self._currentFn.block.buildStore(slotTy, res, slot)
+            if !case.terminator {
+              if res |res| {
+                self._currentFn.block.buildStore(slotTy, res, slot)
+              }
             }
           }
         }

projects/compiler/src/typechecker.abra

@@ -2997,7 +2997,10 @@ export type Typechecker {
           typedNodes.push(typedNode)
           continue
         }
-        _ => try self._typecheckStatement(node: node, typeHint: None)
+        _ => {
+          if self.currentScope.terminator return Err(TypeError(position: node.token.position, kind: TypeErrorKind.UnreachableCode))
+          try self._typecheckStatement(node: node, typeHint: None)
+        }
       }
 
       typedNodes.push(typedNode)
@@ -3231,7 +3234,9 @@ export type Typechecker {
     val terminator = self.currentScope.terminator
 
     self.currentScope = prevScope
-    self.currentScope.terminator = terminator
+    if terminator == Some(Terminator.Returning) {
+      self.currentScope.terminator = terminator
+    }
 
     Ok(TypedAstNode(token: token, ty: Type(kind: TypeKind.PrimitiveUnit), kind: TypedAstNodeKind.While(typedCondition, conditionBindingPattern, typedNodes, terminator)))
   }
@@ -3342,7 +3347,9 @@ export type Typechecker {
     val terminator = self.currentScope.terminator
 
     self.currentScope = prevScope
-    self.currentScope.terminator = terminator
+    if terminator == Some(Terminator.Returning) {
+      self.currentScope.terminator = terminator
+    }
 
     val kind = TypedAstNodeKind.For(typedIterator, (itemPattern, variables), indexBinding, typedNodes)
     Ok(TypedAstNode(token: token, ty: Type(kind: TypeKind.PrimitiveUnit), kind: kind))
@@ -4535,7 +4542,10 @@ export type Typechecker {
       }
     }
 
-    val returnType = if fn.returnType.containsGenerics() {
+    val returnType = if fn.decorators.find(d => d.label.name == "noreturn") {
+      self.currentScope.terminator = Some(Terminator.Returning)
+      Type(kind: TypeKind.Never)
+    } else if fn.returnType.containsGenerics() {
       fn.returnType.withSubstitutedGenerics(resolvedGenerics: resolvedGenerics, retainUnknown: false, genericsInScope: genericsInScope)
     } else {
       fn.returnType

projects/std/src/libc.abra

@@ -49,3 +49,6 @@ export func uname(utsnameBuf: Pointer<Byte>): Int
 
 @CBinding("rand")
 export func rand(): Int
+
+@CBinding("exit")
+export func exit(status: Int): Unit

projects/std/src/prelude.abra

@@ -933,6 +933,9 @@ type Process {
 
     Uname(sysname: sysname, nodename: nodename, release: release, version: version, machine: machine)
   }
+
+  @noreturn
+  func exit(status = 1) = libc.exit(status)
 }
 
 type SetIterator<T> {