Goals The Kestrel Programming Language Part of the documentation for the Kestrel language by Douglas W. Jones THE UNIVERSITY OF IOWA Department of Computer Science

Index

• Goals

Goals

The Kestrel programming language is a refinement of Falcon, a language designed in late 2012 as a target language for a compiler construction course. Therefore, one of its primary goals is compactness. Among its other goals:

• Kestrel, following in the footsteps of the Turing programming language, attempts to eliminate the need for extraneous punctuation and syntactic junk. As in Turing, therefore, semicolons are not required, and the main program need not be delimited by any foolish syntax.

  putstr( "Hello World!"+LF, output )
  

• In Kestrel, the type of bracket, brace or parenthesis used does not imply any semantic categories. Opening and closing delimiters must match, but all grouping delimiters may be used, as in standard mathematical notation. Thus, [(a+b)/(c+d)] is a legal expression and if a is a function of one paramter, a[i] is a legal function call.

• In Kestrel, a number of errors in the design of the C programming language are avoided:

  Arrays are a type distinct from pointers, and array subscripting is not equivalent to pointer arithmetic, following the lead of Pascal.

  Leading zeros on numeric constants do not indicate a change of number base; instead, the number sign # is used, following the lead of Ada.

  Breaking out of the middle of a loop is done by raising an exception, and cases of a case-select construct cannot flow into each other, eliminating the need for a break statement. (The C break statement cost AT&T a huge amount on Jan. 15, 1990 because of a system-wide long-distance system shutdown that was ultimately attributed to the dual use of break in C as both a loop exit and the final statment of each case in a case-select construct.)

• The distinction between assignment, comparison and pointer comparison is inherently messy. In Kestrel, the = operator is used for both assignment and comparison, distinguished by context. Within an expression, it is a comparison operator, while at the top level in a statement, it is used for assignment. Assignment cannot be embedded in expressions, and an expression standing alone cannot serve as a statement. This avoids the use of awkward constructs such as the == comparison operator of the C programming language or the := assignment operator of Algol 60.

  In Kestrel, if a and b are pointer variables, the assignment a=b copies the pointer, and the comparison a=b compares the pointers. Explicit dereferencing is used to refer to the referenced values, so the assignment a@=b@ copies the value, without modifying any pointers, and the comparison a@=b@ compares the referenced values. This follows the lead of Pascal and avoids decisions made by the designers of Ada.