Scheme Cheat Sheet

24th October 2016

Filed under: Best Practices, Featured
Comments: 1

This article provides a quick reference guide for learning Scheme and commonly used expressions. The target audience is for new NexJ developers who have previous programming experience.

Please refer to the Introduction to Scheme course for more in-depth discussion. Also, within NexJ Studio, you can mouse-over each function for details.

Primitives

Description

Scheme example

Java equivalent

Boolean true

true

Boolean false

false

Null

()

null

Integer

100

String

“John”

Timestamp

(parse-date "30-01-2016" "UTC")

; Current Time
(now)

(parse-date "30-01-2016" "UTC")

; Current Time

(now)

Timestamp.valueOf("2016-1-30 00:00:00")

// Current Time
new Timestamp(new Date().getTime())

Timestamp.valueOf("2016-1-30 00:00:00")

// Current Time

new Timestamp(new Date().getTime())

Binary

#z1234567890ABCDEF

new byte[] {18,52,86,120,-112,-85,-51,-17};

Character

#\A

‘A’

Symbol

‘symbol

Declaration and Assignment

Description

Scheme example

Java equivalent

Global scope variable

(define a 1)

1	(define a 1)

int a = 1;

1	int a = 1;

Assign value to variable

(set! a 2)

1	(set! a 2)

a = 2;

a = 2;

Limited scope variable

(let
   (
      (a 1)
      (b 2)
   )
   (+ a b)
)

(let

(

(a 1)

(b 2)

)

(+ a b)

)

{
   int a = 1;
   int b = 2;
   return a + b;
}

{

int a = 1;

int b = 2;

return a + b;

}

Limited scope variable with reference to previously defined variable

(let*
   (
      (a 1)
      (b (+ a 1))
   )
   (+ a b)
)

(let*

(

(a 1)

(b (+ a 1))

)

(+ a b)

)

{
   int a = 1;
   int b = a + 1;
   return a + b;
}

{

int a = 1;

int b = a + 1;

return a + b;

}

Boolean logic

Description

Scheme example

Java equivalent

Not

(not #t)

(not #t)

(! true)

(! true)

And

(and #t #f)

1	(and #t #f)

(true && false)

1	(true && false)

(or #t #f)

1	(or #t #f)

(true || false)

1	(true \|\| false)

Null Check

(null? obj)

1	(null? obj)

(obj == null)

1	(obj == null)

Equal Check

(= a b)
; See eq? eqv? & equal?

1 2	(= a b) ; See eq? eqv? & equal?

(a == b)

(a == b)

Not Equal Check

(!= a b)

; Alternative
(not (= a b))

(!= a b)

; Alternative

(not (= a b))

(a != b)

(a != b)

Conditionals

Description

Scheme example

Java equivalent

If statement

(when (= a b)
   (set! a 1)
   (set! b 1)
)

(when (= a b)

(set! a 1)

(set! b 1)

)

if (a == b) {
   a = 1;
   b = 1;
}

if (a == b) {

a = 1;

b = 1;

}

If…else statement (single line body)

(if (= a b)
   (set! a 1)
   ; else
   (set! b 1)
)

(if (= a b)

(set! a 1)

; else

(set! b 1)

)

if (a == b)
   a = 1;
else
   b = 1;

if (a == b)

a = 1;

else

b = 1;

If…else statement (multi-line body)

(cond
   ((= a b)
      (set! a 1)
      (set! b 1)
   )
   (else
      (set! a 2)
      (set! b 2)
   )
)

(cond

((= a b)

(set! a 1)

(set! b 1)

)

(else

(set! a 2)

(set! b 2)

)

if (a == b) {
   a = 1;
   b = 1;
} else {
   a = 2;
   b = 2;
}

if (a == b) {

a = 1;

b = 1;

} else {

a = 2;

b = 2;

}

Multi-condition if statement

(cond
   ((>= x 5) "High")
   ((>= x 0) "Low")
   (else "Negative")
)

(cond

((>= x 5) "High")

((>= x 0) "Low")

(else "Negative")

)

String ret;
if (x >= 5)
   ret = "High";
else if (x >= 0)
   ret =  "Low";
else
   ret =  "Negative";

String ret;

if (x >= 5)

ret = "High";

else if (x >= 0)

ret = "Low";

else

ret = "Negative";

Ternary Operator

(if (= a b) 1 2)

1	(if (= a b) 1 2)

(a == b) ? 1 : 2;

1	(a == b) ? 1 : 2;

Non-Boolean Condition

(define condition ())
(if condition
   "A"
   ;else
   "B"
)
; returns "A"

; Caution: All non-#f values evaluates to #t

(define condition ())

(if condition

"A"

;else

"B"

)

; returns "A"

; Caution: All non-#f values evaluates to #t

Boolean condition = null;
if (condition)
   return "A";
else
   return "B";
 
// Throws Null Pointer Exception at run-time

Boolean condition = null;

if (condition)

return "A";

else

return "B";

// Throws Null Pointer Exception at run-time

Collections

Description

Scheme example

Java equivalent

Collection

(define col
(collection 5 6 7)
)

(define col

(collection 5 6 7)

)

List col = new ArrayList();
col.add(5);
col.add(6);
col.add(7);

// Alternative
List col = Arrays.asList(5,6,7);

List col = new ArrayList();

col.add(5);

col.add(6);

col.add(7);

// Alternative

List col = Arrays.asList(5,6,7);

Collection Lookup by Index

(col'get 1)
; Returns 6

1 2	(col'get 1) ; Returns 6

col.get(1);

1	col.get(1);

Collection Length

(col'size)
; Returns 3

1 2	(col'size) ; Returns 3

col.size();

1	col.size();

Collection Empty Check

(col'empty)

1	(col'empty)

col.isEmpty();

1	col.isEmpty();

List

(define lst (list 5 6 7))

; Alternative
(define lst '(5 6 7))

(define lst (list 5 6 7))

; Alternative

(define lst '(5 6 7))

List Lookup by Index

(list-ref lst 1)
; Returns 6

1 2	(list-ref lst 1) ; Returns 6

List Length

(length lst)

; Returns 3

(length lst)

; Returns 3

List Empty Check

(null? lst)

1	(null? lst)

Collection/List Lookup by Value

(collection-find-item
'(5 6 7)
(= (@) 6)
)
; Returns 6
; Caution: Returns #f if no match

(collection-find-item

'(5 6 7)

(= (@) 6)

)

; Returns 6

; Caution: Returns #f if no match

Arrays.asList(5, 6, 7)
.stream()
.filter(x -> x == 6)
.findAny()
.orElse(false);

Arrays.asList(5, 6, 7)

.stream()

.filter(x -> x == 6)

.findAny()

.orElse(false);

Collection/List Filtering

(collection-find-items
'(5 6 7)
(>= (@) 6)
)
; Returns '(6 7)
; Returns () if no match

(collection-find-items

'(5 6 7)

(>= (@) 6)

)

; Returns '(6 7)

; Returns () if no match

Arrays.asList(5, 6, 7)
.stream()
.filter(x -> x >= 6)
.collect(Collectors.toList());

Arrays.asList(5, 6, 7)

.stream()

.filter(x -> x >= 6)

.collect(Collectors.toList());

Collection/List Sorting

(sort
(collection 5 6 7)
>
)
; Returns (collection 7 6 5)

(sort

(collection 5 6 7)

)

; Returns (collection 7 6 5)

Arrays.asList(5, 6, 7)
.stream()
.sorted((x, y) -> y.compareTo(x))
.collect(Collectors.toList());

Arrays.asList(5, 6, 7)

.stream()

.sorted((x, y) -> y.compareTo(x))

.collect(Collectors.toList());

Loops

Description

Scheme example

Java equivalent

For loop through collection/list

(for-each
(lambda (i)
(display i)
)
(collection 1 2 3)
)

(for-each

(lambda (i)

(display i)

)

(collection 1 2 3)

)

for (int i : Arrays.asList(1, 2, 3)) {
System.out.print(i);
}

for (int i : Arrays.asList(1, 2, 3)) {

System.out.print(i);

}

For loop using index

(for
((i 0))
(< i 3)
(set! i (+ i 1))
(display i)
)

; Alternative
(for-each
(lambda (i)
(display i)
)
(range 3)
)

(for

((i 0))

(< i 3)

(set! i (+ i 1))

(display i)

)

; Alternative

(for-each

(lambda (i)

(display i)

)

(range 3)

)

for (int i = 0; i < 3; i++) {
System.out.print(i);
}

for (int i = 0; i < 3; i++) {

System.out.print(i);

}

Functions

Description

Scheme example

Java equivalent

Declare function

(define (myFunc p1 p2)
(display p1)
(display p2)
; return
(+ p1 p2)
)

; Alternative
(define myFunc
(lambda (p1 p2)
(display p1)
(display p2)
; return
(+ p1 p2)
)
)

(define (myFunc p1 p2)

(display p1)

(display p2)

; return

(+ p1 p2)

)

; Alternative

(define myFunc

(lambda (p1 p2)

(display p1)

(display p2)

; return

(+ p1 p2)

)

Caution: Last expression is always the return value

public static int myFunc(int p1, int p2) {
System.out.print(p1);
System.out.print(p2);
return p1 + p2;
}

public static int myFunc(int p1, int p2) {

System.out.print(p1);

System.out.print(p2);

return p1 + p2;

}

Invoke function

(myFunc 1 2)

1	(myFunc 1 2)

myFunc(1, 2);

1	myFunc(1, 2);

Declare function with variable arguments

(define
(myFunc p1 p2 . args)
; return
(+
p1
p2
(list-ref args 0)
(list-ref args 1)
)
)

(define

(myFunc p1 p2 . args)

; return

(list-ref args 0)

(list-ref args 1)

)

public static int myFunc(

int p1,

int p2,
int... args)
{
return p1
+ p2
+ args[0]
+ args[1];
}

public static int myFunc(

int p1,

int p2,

int... args)

{

return p1

+ p2

+ args[0]

+ args[1];

}

Invoke function with variable arguments

(myFunc 1 2 3 4)

; Alternative
(apply myFunc
1 2 (collection 3 4)
)

(myFunc 1 2 3 4)

; Alternative

(apply myFunc

1 2 (collection 3 4)

)

myFunc(1, 2, 3, 4)

1	myFunc(1, 2, 3, 4)

Function Documentation

; This function adds 2 numbers together
; @arg p1 int First Num
; @arg p1 int Second Num
; @ret int Sum of p1 and p2
; @example (myFunc 4 5) => 9
(define (myFunc p1 p2)
; return
(+ p1 p2)
)

; This function adds 2 numbers together

; @arg p1 int First Num

; @arg p1 int Second Num

; @ret int Sum of p1 and p2

; @example (myFunc 4 5) => 9

(define (myFunc p1 p2)

; return

(+ p1 p2)

)

/**
* This function add 2 numbers together
* @param p1 First Num
* @param p2 Second Num
* @return Sum of p1 and p2
*/
public static int myFunc(int p1, int p2)

/**

* This function add 2 numbers together

* @param p1 First Num

* @param p2 Second Num

* @return Sum of p1 and p2

public static int myFunc(int p1, int p2)

Objects

Description

Scheme example

Java/SQL equivalent

Object instantiation

(define psn (Person'new))

1	(define psn (Person'new))

Person psn = new Person();

1	Person psn = new Person();

Object instantiation with arguments

(define psn
(Person'new
(: firstName "John")
(: lastName "Smith")
)
)

(define psn

(Person'new

(: firstName "John")

(: lastName "Smith")

)

Person psn = new Person("John", "Smith");

1	Person psn = new Person("John", "Smith");

Object Update

(psn'lastName "Paul")

1	(psn'lastName "Paul")

psn.setLastName("Paul");

1	psn.setLastName("Paul");

Object Delete

(psn'delete)

1	(psn'delete)

psn.delete();

1	psn.delete();

Object Association

((((psn'company)'addresses)'get 0)'city)

; See @ macro for short form

((((psn'company)'addresses)'get 0)'city)

; See @ macro for short form

psn.company.addresses[0].city;

1	psn.company.addresses[0].city;

Single Object Read

(read-instance
Person
'(firstName)
'(= lastName "Smith")
()
)
; Returns a single instance

(read-instance

Person

'(firstName)

'(= lastName "Smith")

()

)

; Returns a single instance

Caution: Throws error if more than 1 record exists

SELECT A.id, A.firstName
FROM NJEntity A
WHERE A.lastName = ?

SELECT A.id, A.firstName

FROM NJEntity A

WHERE A.lastName = ?

Multi Object Read

(Person'read
'(firstName)
'(= lastName "Smith")
()
()
()
()
)
; Returns a collection of Person objects

(Person'read

'(firstName)

'(= lastName "Smith")

()

)

; Returns a collection of Person objects

SELECT A.id, A.firstName
FROM NJEntity A
WHERE A.lastName = ?

SELECT A.id, A.firstName

FROM NJEntity A

WHERE A.lastName = ?

Logging

Description

Scheme example

Java equivalent

Info-level logging

(logger'info "Value:" 100)

1	(logger'info "Value:" 100)

System.out.println("Value:" + " " + 100);

1	System.out.println("Value:" + " " + 100);

Debug-level logging

(logger'debug "Message")

1	(logger'debug "Message")

System.out.println("Message");

1	System.out.println("Message");

Error-level logging

(logger'error ex)

1	(logger'error ex)

System.err.println(ex);

1	System.err.println(ex);

Error Handling

Description

Scheme example

Java equivalent

Raise error

(error "err.invalidParameter")

1	(error "err.invalidParameter")

throw new Exception("Invalid Parameter");

1	throw new Exception("Invalid Parameter");

Assertion error

(assert (not (null? p1)))

1	(assert (not (null? p1)))

assert (p1 != null);

1	assert (p1 != null);

Try..Catch..Finally

(try
; body
(begin
(display "Body")
(error "err.badParam")
)
; catch
(lambda (e)
(display e)
)
; finally
(display "Finally")
)

(try

; body

(begin

(display "Body")

(error "err.badParam")

)

; catch

(lambda (e)

(display e)

)

; finally

(display "Finally")

)

try {
System.out.print("Body");
   throw new Exception("err");
} catch (Exception e) {
System.err.print(e);
} finally {
System.out.print("Finally");
}

try {

System.out.print("Body");

throw new Exception("err");

} catch (Exception e) {

System.err.print(e);

} finally {

System.out.print("Finally");

}

Try…Catch with restricted handler

(try
; body
(begin
(display "Body")
(error "err.badParam")
)
; restricted catch
(catch ex
("err.badParam"
(display ex)
)
)
)

(try

; body

(begin

(display "Body")

(error "err.badParam")

)

; restricted catch

(catch ex

("err.badParam"

(display ex)

)

try {
System.out.print("Body");
   throw new Exception("err");
} catch (Exception e) {
String msg = e.getMessage();
   if (msg.equals("err"))
System.err.print(e);
else
      throw e;
}

try {

System.out.print("Body");

throw new Exception("err");

} catch (Exception e) {

String msg = e.getMessage();

if (msg.equals("err"))

System.err.print(e);

else

throw e;

}

Scheme Commands for Developers

Description

Scheme example

Reset Scheme Console after metadata hotswap

(*repl*'reset)

1	(repl'reset)

Turn up SQL logging

(debug 'sql)

; Alternative (if debugScripts.scm is not available)
(import '(org.apache.log4j.Logger org.apache.log4j.Level))
(
(
org.apache.log4j.Logger'getLogger
"nexj.core.persistence.sql.SQLAdapter"
)
'level
(org.apache.log4j.Level'DEBUG)
)

(debug 'sql)

; Alternative (if debugScripts.scm is not available)

(import '(org.apache.log4j.Logger org.apache.log4j.Level))

(

org.apache.log4j.Logger'getLogger

"nexj.core.persistence.sql.SQLAdapter"

)

'level

(org.apache.log4j.Level'DEBUG)

)

Turn up RPC logging

(debug 'rpc)

; Alternative (if debugScripts.scm is not available)
(import '(org.apache.log4j.Logger org.apache.log4j.Level))
(
(
org.apache.log4j.Logger'getLogger
"nexj.core.rpc.GenericServer"
)
'level
(org.apache.log4j.Level'ALL)
)

(debug 'rpc)

; Alternative (if debugScripts.scm is not available)

(import '(org.apache.log4j.Logger org.apache.log4j.Level))

(

org.apache.log4j.Logger'getLogger

"nexj.core.rpc.GenericServer"

)

'level

(org.apache.log4j.Level'ALL)

)

Reseed Enumerations only

; Dependent on debugScripts.scm
(reseed 'enums)

1 2	; Dependent on debugScripts.scm (reseed 'enums)

NexJ Studio Shortcuts

Description	Shortcuts
Open file by name	CTRL + SHIFT + D This dialog includes files inherited from base models.
Open file by name	CTRL + SHIFT + R This dialog excludes files from base models.
File search by file content	CTRL + H Switch to “Model Search” tab
Execute selected text in Scheme Console	CTRL + U
Select text between matching brackets	CTRL + SHIFT + \
Auto code format	CTRL + SHIFT + F
Code auto-complete	CTRL + SPACE
Auto-indent as you type	Window -> Preferences -> NexJ Studio -> Check “Auto-indent Scheme script as you type”
Delete current line	CTRL + D
Block commenting	CTRL + SHIFT + /
Rename variable	ALT + SHIFT + R

24th October 2016

Primitives

Declaration and Assignment

Boolean logic

Conditionals

Collections

Loops

Functions

Objects

Logging

Error Handling

Scheme Commands for Developers

NexJ Studio Shortcuts

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