Conditionals

Quick reference

It is useful to have the flow of your code run through alternative paths based on whether certain conditions are met. To do this logical flow control you use conditionals which include the commands: if, when, unless, and case.

If you think that you’ve heard this story before – please read on – SkookumScript may have some surprises for you.

Other mechanisms for alternate paths
Closures – especially routines that take closures as arguments – concurrent blocks, and the short-circuit Boolean operators and, or, nand and nor can also be used to create alternate code paths.

‘if’ expression

An if expression has one or more test expressions that evaluate to a Boolean (true or false) paired with associated clause blocks (sometimes called then clauses). Whichever test expression evaluates to true first will run its associated clause block and then skip any remaining test/clause pairs and continue on to the next expression following the entire if expression.

The simplest form of an if expression is a single test expression with a single clause block.

Single Test/Clause Pair

// Test expressions and clauses can be simple
if expr > 5 [do_this]

// Method calls that return true/false usually end with '?'
// The fancy name for this is a 'predicate' method.
if test? [do_this]

if not test? [do_this]

// Test expressions can have multiple conditions
if test1? and test2? [do_this]

// Clauses can have multiple expressions
if test? [do_this1 do_this2]

if test?
  [
  do_this1
  do_this2
  ]

Simple conditionals with a single expression clause are often best represented with statement modifiers – see when and unless.

Often you will want to have a test expression that always succeeds and ensures that at least one clause block is run. On these occasions you use an else clause.

An else is essentially the same as a test expression of true – though it looks a little nicer and more explicitly gets your intent across.

‘else’ Clause

if test? [do_this] else [do_that]

// Essentially the same as:
if test? [do_this] true [do_that]

// The spacing and layout can be in many forms.
if test? [do_this]
  else [do_that]

if test?
  [do_this]
else
  [do_that]

if test?
  [
  do_this
  ]
else
  [
  do_that
  ]

// An else clause can have several expressions
// just like other clauses.
if test?
  [
  do_this1
  do_this2
  ]
else
  [
  do_that1
  do_that2
  ]

Pro Tip: Two Random Alternate Paths
If you want to randomly alternate between two possible code paths use the Random.coin_toss() method which randomly returns true or false.

if @@random.coin_toss [path1] else [path2]

@@random is the SkookumScript default psuedo-random number genenerator object which is always available.

Here is where things get a little interesting compared to many other languages. SkookumScript supports multiple clauses in a single if expression. Just chain as many test/clause pairs together as you like and end it with an optional else clause.

The if expression is somewhat similar to the Lisp cond expression though it has its own syntax. The other conditional expressions have similar usage and naming to Lisp conditionals as well. This is cool – though actually a total coincidence and an example of convergent evolution.

Multi-clause

if test1? [do_one]
  test2?  [do_two]
  else    [do_else]

if test1?
  [do_one]
test2?
  [do_two]
else
  [do_else]

if test1?
  [
  do_one1
  do_one2
  ]
test2?
  [
  do_two1
  do_two2
  ]
else
  [
  do_else1
  do_else2
  ]

// If you are familiar with other languages that
// do not allow mulitple clauses it means that
// code like this below is a thing of the past:
if test1?
  [do_one]
else
  [
  if test2?
    [do_two]
  else
    [
    if test3?
      [do_three]
    else
      [do_else]
    ]
  ]

// Isn't this better?
if test1?
    [do_one]
  test2?
    [do_two]
  test3?
    [do_three]
  else
    [do_else]

‘when’ expression modifier

Simple conditionals with a single clause can also be represented with the when expression modifier. when does not require a code block around its clause. It essentially makes an optional expression.

do_this when test?

// Same as code
if test? [do_this]

do_that when expr > 5

obj.do_stuff when obj.valid?

Pro Tip: Random Expression
You can easily make an expression run randomly with Random.coin_toss() and when.

do_this when @@random.coin_toss

You *can* use a code block to group expressions in a when clause, though generally you should use an if in these cases instead and reserve when for simple single expressions.

// Legal - but not recommended
[do_this do_that do_stuff] when test?

// Prefer `if` for clauses with multiple expressions
if test?
  [
  do_this
  do_that
  do_stuff
  ]

‘unless’ expression modifier

unless is an expression modifier similar to when that negates the test and only executes the earlier clause expression if the later test expression is false.

do_this unless test?

// Same as code
if not test? [do_this]

do_that unless expr > 5

obj.do_stuff unless obj.invalid?

As with a when expression, you *can* use a code block to group expressions in an unless clause, though generally you should use an if and not in these cases instead and reserve unless for simple single expressions.

‘case’ expression

A case expression is similar to an if expression except that it does a logical equal?()/= comparison with an initial expression and a bunch of potential value expression and clause pairs. There can also be an optional else clause.

The initial expression is evaluated and then equal?()/= is called on it with each potential value in succession until one matches (returns true). If there are no matches then any optional else clause is run. Then the control flow continues on to the next expression following the entire case expression.

Several other languages use the key word switch rather than case. Also some languages use default rather than else. SkookumScript uses else like the if expression for consistency.

‘case’ expression

case expr
  val1 [do_one]
  val2 [do_two]
  else [do_three]

case expr
  val1
    [
    do_one1
    do_one2
    ]
  val2
    [
    do_one1
    do_one2
    ]
  else
    [
    do_three1
    do_three2
    ]

Common conditional considerations

All conditionals are expressions that can return a result and they all can bind variables in their clauses which may change the types of the variables.

Using the result of conditionals

The language geeks among you may have raised an eyebrow when I said conditional expression rather than statement – this is not a mistake – all conditionals are an expression and all expressions return a value.

Just like other code blocks, the last expression in the successful clause will be the result. If there is no successful clause and there is no else clause then then result will be nil. Usually only conditionals where all paths have a result (and not nil) are used as a result – though even conditionals that may return nil can be used.

Because the result of a conditional expression will be one of several alternative paths and the parser cannot know ahead of time which path will be successful, the class type of the result will be a union of all the result types from each of the possible paths.

Result of an ‘if’ expression

// Single clause
!result: if test? ["first"]
// result will be "first" or nil
// and its class type will be <String|None>

// Single clause with else clause
!result: if test? ["first"] else ["else"]
// result will be "first" or "else"
// and its class type will be String
// All the conditional paths have the same result type
// so there is no need for a union class.

// Single clause with else clause and different types
!result: if test? ["first"] else [42]
// result will be "first" or 42
// and its class type will be <String|Integer>

// Multiple clauses - same type
!result: if test1? ["one"] test2? ["two"] else ["else"]
// result will be "one", "two" or "else"
// and its class type will be String

// Multiple clauses - different types
!result: if test1? [42] test2? [2.0] else ["else"]
// result will be 42, 2.0 or "else"
// and its class type will be <Integer|Real|String>

// The result of an if can also be used as an argument
println(if success? ["success"] else ["failure"])

if expressions are essentially the same as the ternary operator used by many languages like ?: in C++ – though the SkookumScript if can have multiple clauses and has no statement (no result) version.

Result of modifier expressions

!result: "first" when test?
// result will be "first" or nil
// and its class type will be <String|None>

// Can also be used as an argument
println(obj.name unless obj.invalid?)

Result of a ‘case’ expression

// Single clause
!result: case expr val1 ["first"]
// result will be "first" or nil
// and its class type will be <String|None>

// Single clause with else clause
!result: case expr val1 ["first"] else ["else"]
// result will be "first" or "else"
// and its class type will be String

// Single clause with else clause and different types
!result: case expr val1 ["first"] else [42]
// result will be "first" or 42
// and its class type will be <String|Integer>

// Multiple clauses - same type
!result: case expr val1 ["one"] val2 ["two"] else ["else"]
// result will be "one", "two" or "else"
// and its class type will be String

// Multiple clauses - different types
!result: case expr val1 [42] val2 [2.0] else ["else"]
// result will be 42, 2.0 or "else"
// and its class type will be <Integer|Real|String>

// The result of a case can also be used as an argument
println(
  case guy.name
    'Vader' ["bad guy"]
    'Yoda'  ["good guy"]
    else    ["don't know"]
  )

Binding variables in clauses

Variables can be rebound (:) to different class types in clause code blocks. Since the parser cannot know which clause code path will be successful, the type of a varable after a conditional is a union of all the possible binding types.

Type Changing in Clauses

!val: 42

// Type of val is Integer

if test? [val: 1234]

// Type of val is still Integer

if test? [val: "[Sk] can do fancy type-checking!"]

// Type of val is <Integer|String>

if test?
  [val: 2.0]
else
  [val: 3.14]

// Type of val is Real

if test1?
    [val: 888]
  test2?
    [val: "[Sk] tracks types for you"]

// Type of val is <Integer|Real|String>