Control Structures

Single Line and Multi-Line Commands

if, while, and forEach support single line commands as well as multi-line commands.
If a control structure has commands on the same line after the condition, it is taken to be a single line statement - all commands on that line (separated by semi-colons) are part of the body and no terminal end marker is required.
If a control structure doesn't have commands immediately following the condition, it's considered a multi-line command and an appropriate "end" command is looked for.
A multi-part if/elseIf/else cannot mix and match single and multi-line blocks. They must all be single or all be multi (otherwise nested if statements could be ambiguous).
Single line statements may not have any nested control structures.

If

Syntax:

if (condition)
  ...
elseIf (condition)
  ...
else
  ...
endIf

if (condition) single; line; commands
elseIf (condition) single; line; commands
else single; line; commands

Examples:

if (x > 0) println( "$ is positive" (x) )
elseIf (x < 0) println( "$ is negative" (x) )
else println( "0 is zero" )

Like all conditionals in Slag, if requires parentheses around the condition.
escapeIf skips to the command after the enclosing endIf.

Which

Examples:

which (ch)
  case  9: return "TAB"
  case 10, 13: return "EOL"  # well, kinda...
  case 27: return "ESC"
  case 32..126: return ""+Char(ch)
  others: return "^" + Char(ch+64)
endWhich

forEach (a in 1..2)
  forEach (b in 1..2)
    which (a,b)
      case (1,1): println( "R" )
      case (1,2): println( "S" )
      case (2,1): println( "T" )
      case (2,2): println( "U" )
    endWhich
  endForEach
endForEach

The which structure replaces the switch or select of other languages.
Its cases may be any expressions, not just constants.
Internally which statements are converted into if statements. The expression is saved in a local variable and then compared to each case value.
No "break" commands are necessary.
"others:" replaces "default:"
Independent values can be part of the same case when separated by commas ("case a,b,c:" ).
A case may contain a simple range of values, e.g. "case 1..10:". This is converted into a two-part conditional ("if (temp_value >= 1 and temp_value <= 10)").
escapeWhich skips to the command after the enclosing endWhich.

Contingent

Syntax:

contingent
  # body (regular commands)
  ...
  necessary (condition)
  ...
  sufficient (condition)
  ...
satisfied
  # commands if execution reaches end of body or any 'sufficient' cmd true 
unsatisfied
  # commands if any 'necessary' tests false.
endContingent

Example:

method print_primes( Int32 low, Int32 high ):
  forEach (n in low..high)
    contingent
      necessary (n >= 2)
      sufficient (n == 2)
      forEach (d in {2,3..sqrt(n) step 2})
        necessary (n % d != 0)
      endForEach
    satisfied
      println( "$ is a prime number" (n) )
    endContingent
  endForEach

Slag introduces contingents as a new kind of conditional. They are similar in flow to a try/catch block but they deal with positive and negative logic tests rather than error generation.
They are useful when it will take several consecutive commands to determine whether to execute "success" or "failure" code.
Any commands may be placed in the body. These commands can include necessary() and sufficient() logic tests.
If a necessary condition is true, execution proceeds to the next command. If false, execution jumps to the unsatisfied clause.
If a sufficient condition is false, execution proceeds. If true, execution jumps to the satisfied clause.
If all the body code is executed with no necessary conditions being false and no sufficient conditions being true, execution proceeds with the satisfied clause.
If the satisfied clause executes, the unsatisfied clause is skipped.
The satisfied and unsatisfied clauses are both optional. If not present the contingent behaves as if they were empty.
escapeContingent skips to the command after the enclosing endContingent.

While

Syntax:

while (condition)
  ...
endWhile

while (condition) single; line; commands

Example:

# Sum together the digits of a non-negative integer until one digit is left.
while (n > 0)
  n = n/10 + n%10
endWhile

escapeWhile skips to the command after the enclosing endWhile.
nextIteration skips the remainder of the current iteration and proceeds onto the next iteration.

Loop

Syntax:

loop
  ...
endLoop

Example:

# Keep reading in numbers from some reader until a zero is read.
loop
  local var n = reader.read
  if (n == 0) escapeLoop
  list.add(n)
endLoop

loop loops indefinitely and is a convenient alternative to "while (true) ...".
escapeLoop skips to the command after the enclosing endLoop.
nextIteration skips the remainder of the current iteration and proceeds onto the next iteration.

ForEach

Syntax:

# multi-line variants
forEach (range)
  ...
endForEach

forEach (var_name in range)
  ...
endForEach

forEach (var_name of range)
  ...
endForEach

# single line variants
forEach (range) single; line; commands

forEach (var_name in range) single; line; commands

forEach (var_name of range) single; line; commands

Examples:

forEach (1..100) println("I will use iteration to make my life easier.")

forEach (line in LineReader(File("settings.txt"))) println(line)

forEach (ch in "nilbog".reverse_order) print(ch)
println

forEach (entity in mobs)
  entity.update
  if (entity.is_dead) removeCurrent entity
endForEach

method index_of( Real64 n ).Int32:
  forEach (index of data)
    if (data[index] == n) return index
  endForEach
  return -1

escapeForEach skips to the command after the enclosing endForEach.
nextIteration skips the remainder of the current iteration and proceeds onto the next iteration.
Read the notes on advanced forEach usage to learn about:
- How forEach loops are processed internally.
- How to write classes that are compatible with forEach.
- What causes concurrent modification errors and how to avoid them.
- How the removeCurrent command works.

Try/Catch & Throw

Syntax:

try
  ...
catch (MostSpecificType err)
  ...
catch (LessSpecificType err)
  ...
catch (MostGeneralType err)
  ...
endTry

...
throw Error("badwrong")

Example:

try
  load_save_data( File("save.txt") )
catch (FileError err)
  set_up_default_data()
endTry

The throw command takes an Exception (or subclass) object and unwinds the call stack to the most recently-defined catch of the correct type, no matter many nested calls you're currently in.
try/catch and throw operate with objects extended from class Exception.
Class Error extends class Exception and all built-in exceptions are extended from type Error.
There are no checked exceptions in Slag - errors may be thrown at any time (without writing a declaration in the method signature) and enclosing try/catch blocks are entirely optional.
Exception objects that are instanceOf the first catch type are handled by that catch and the other catches are skipped.
Slag does not have a finally clause.

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Contents

Control Structures

Single Line and Multi-Line Commands

If

Which

Contingent

While

Loop

ForEach

Try/Catch & Throw