SCL Tutorial

Getting started

The easiest way of getting started with SCL is to use SCL console that is included in almost all Simantics-based products. You can open the console by pressing ALT-SHIFT-q and then q and choosing "SCL Console" from the list of views.

SCL console works by executing commands you write into the input box in the bottom of the view. After the command has been written, it can be executed by pressing ENTER. However, this works only if the command contains no syntactic errors. Possible errors are highlighted in the input box and a description of the error is shown when you move mouse on top of the highlighted text.

Multiline commands can be written by pressing CTRL-ENTER (or just ENTER when the current command text contains errors). The command history can be browsed with CTRL-UP and CTRL-DOWN.

If the command you write into console results as an ordinary value, it is just printed to the console. Here are couple of examples you can try:

> 13
13
> 1+2
3
> sin 1
0.8414709848078965
> "Hello " + "world!"
Hello world!
> [1,3,5]
[1, 3, 5]

You can also declare local variables to be used in the commands:

> x = 35
> y = 40
> x + y
75
> x * y
1400

Also new functions can be defined:

> f x = x * x
> f 123
15129

If you write a command that has side-effects. It is executed in the console:

> print "Hello" ; print "world!"
Hello
world!

SCL is a dialect of Haskell and tutorials written for Haskell can be used for learning the details of the language. The main differences between the languages are the strict evaluation strategy used in SCL and somewhat different standard library. Some Haskell tutorials can be found at http://www.haskell.org/haskellwiki/Learning_Haskell.

Extending SCL environment

The SCL values, data types etc. that are available in expressions and commands are defined in SCL modules. Currently all SCL modules must be part of the product plugins (in the future, you can also write modules inside the models). Each module is identified by a URI.

SCL module is a text file ending with extension ".scl". The recommended place for modules is scl/ folder under plugin root, but also other directories can be used:

scl/Test1.scl:

fib :: Integer -> Integer
fib x | x <= 1    = 1
      | otherwise = fib (x-1) + fib (x-2)

A directory is declared as a SCL package with the following kind of extension points defined in org.simantics.scl.runtime:

   <extension point="org.simantics.scl.runtime.package">
      <package URI="http://www.simantics.org/Tests"
               directory="scl"/>
   </extension> 

The module is not automatically available in the console, but you must run an import declaration:

> import "http://www.simantics.org/Tests/Test1" as Test1
> Test1.fib 13
377

Import declaration can also be used in modules to refer other modules. Cyclic module dependencies are not allowed.

Importing functionality from Java

Java interfaces and classes can be imported to Java with annotated data definitions:

@JavaType "java.util.regex.Pattern"
data Pattern

@JavaType "java.util.List"
data List a

There are SCL macros for calling constructors and methods of the classes and accessing fields. These macros are defined in http://www.simantics.org/Java.

import "http://www.simantics.org/Java" as Java

compilePattern : String -> Pattern
compilePattern = Java.staticMethod "java.util.regex.Pattern.compile"

@JavaType "java.util.regex.Matcher"
data Pattern

createMatcher: Pattern -> String -> Proc Matcher
createMatcher = Java.methodProc "matcher"

matcherMatches: Matcher -> Proc Boolean
matcherMatches = Java.methodProc "matches"

matches : Pattern -> String -> Boolean
matches pattern text = runProc (createMatcher pattern text >>= matcherMatches)    

Another example:

createArrayList : Proc (List a)
createArrayList = Java.constructorProc "java.util.ArrayList"

createArrayListWithCapacity : Integer -> Proc (List a)
createArrayListWithCapacity = Java.constructorProc "java.util.ArrayList"

sizeList : List a -> Proc Integer
sizeList = Java.methodProc "size"

getList : List a -> Integer -> Proc a
getList = Java.methodProc "get"

setList : List a -> Integer -> a -> Proc ()
setList = Java.methodProc "set"

addList : List a -> a -> Proc Boolean
addList = Java.methodProc "add"

Names of classes and methods must be compile time constants. Also the type of the method must be completely defined, because SCL compiler cannot infer the type from the Java method. Java method signature is however checked to be compatible with SCL type. Overloaded methods are handled by filtering out those methods whose signatures do not match.

The complete list of functions for accessing Java classes and interfaces is:

• constructor, constructorProc
• method, methodProc
• staticMethod, staticMethodProc
• field, fieldProc
• staticField, staticFieldProc
• setField

Macro constructor expects the full class name as a parameter. Macros staticMethod and staticField expect the full method or field name including the full name of the class containing the method or field. Macros method, field and setField expect only the name of the method or field without the containing class name.

xxxProc-variants of the macros access Java methods that are not referentially transparent (have side-effects). They expect Proc Something as a return type of the function. Note that although functions staticField and field only read a value of the field, they are not referentially transparent if it is possible that the value of the field changes.

It is common that Java methods that have side-effects can be combined so that resulting methods are side-effect free. SCL function

runProc : Proc a -> a

can be used to declare that the procedure actually is reflectively transparent. See above how it was used to defined matches function. It should be used sparingly in ordinary SCL code, because if it is used for a procedure that is not actually referentially transparent, strange behavior can occur when the compiler reorders or removes dead code.

A major functionality currently still missing is the ability to create new implementations of existing Java interfaces in SCL code or extend an existing class. This can be worked around currently by writing new implementations in Java and creating them with constructor macro.