FCL - Forth Calculator's Language

FCL is the programming language of an Android app called Forth Calculator. It is a Forth dialect with optional local variables, complex data structures, quotations and Java interoperability.

: fib ( n1 n2 -- n1 n2 n3 ) 2dup + ;

: nfib ( n -- .. ) -> n ( local variable )
  0 1 { fib } n times ; ( quotation )

Besides all the high-level features, FCL supports the traditional Forth programming structures and uses the same compilation model (compile/interpret mode, dictionary, immediate words, etc.) as traditional Forth systems.

The Syntax

The syntax is a superset of the Forth language. In FCL there are literal syntax for creaing Lists [ 1 2 3 ], Maps #[ 'key' 'value' ]#, Quotations { dup + } and Strings 'Hello World'.

Control structures

FCL supports the traditional Forth conditional and loop control structures.

General form of if else then.

<bool> if <consequent> else <alternative> then

For example:

: max ( a b -- max )
  2dup < if nip else drop then ;

10 100 max . \ prints 100

The else part is optional.

: abs ( n -- n ) 
  dup 0 < if -1 * then ;

-10 abs . \ prints 10

Case statement

FCL supports switch-case like flow control logic as shown in the following example.

: day ( n -- )
  case
    1 of print: 'Monday' endof
    2 of print: 'Tuesday' endof
    3 of print: 'Wednesday' endof
    4 of print: 'Thursday' endof
    5 of print: 'Friday' endof
    6 of print: 'Saturday' endof
    7 of print: 'Sunday' endof
    drop 'Unknown'
  endcase ;

Count-controlled loops

The limit and start before the word do defines the number of times the loop will run.

<limit> <start> do <loop-body> loop

Do loops iterate through integers by starting at start and incrementing until you reach the limit. The word i pushes the loop index onto the stack. In a nested loop, the inner loop may access the loop variable of the outer loop by using the word j.

For example:

5 0 do i . loop \ prints 0 1 2 3 4

It is important to understand the implementation details of this loop. DO loops store the loop index on the return stack. You can break the semantics of i and j if you use the return stack to store temporary data. Exiting from the loop requires clearing up the return stack by using the unloop word.

Condition-controlled loops

until loop

begin <loop-body> <bool> until

The begin...until loop repeats until a condition is true. This loop always executes at least one time.

For example:

: countdown ( n -- )
  begin
    dup .
    1- dup
    0 < 
  until
  drop ;

5 countdown \ prints 5 4 3 2 1 0

while loop

begin .. <bool> while <loop-body> repeat

For example:

: countdown ( n -- )
  begin
    dup 0 >=
  while
    dup . 1-
  repeat
  drop ;

5 countdown \ prints 5 4 3 2 1 0

Control structres are compile time words with no interpretation semantics.

Locals

Maps

List

Quotations

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