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Text File  |  1991-07-02  |  1KB  |  44 lines
  1.  
  2. <H2><A ID="SECTION00185000000000000000">
  3. The ``Program Feature''</A>
  4. </H2>
  5.  
  6. <P>
  7. Lisp implementations since Lisp 1.5 have had what was originally
  8. called ``the program feature,'' as if it were impossible to write
  9. programs without it!  The <#2704#>prog<#2704#> construct allows one to
  10. write in an Algol-like or Fortran-like statement-oriented
  11. style, using <#2705#>go<#2705#> statements that can refer to tags in the
  12. body of the <#2706#>prog<#2706#>.  Modern Lisp programming style tends to use
  13. <#2707#>prog<#2707#> rather infrequently.  The various iteration constructs,
  14. such as <#2708#>do<#2708#>, have bodies with the characteristics of a <#2709#>prog<#2709#>.
  15. (However, the ability to use <#2710#>go<#2710#> statements within iteration
  16. constructs is very seldom called upon in practice.)
  17.  
  18. <P>
  19. Three distinct operations are performed by <#2711#>prog<#2711#>: it binds local variables,
  20. it permits use of the <#2712#>return<#2712#> statement, and it permits use of the <#2713#>go<#2713#>
  21. statement.
  22. In Common Lisp, these three operations have been separated into three
  23. distinct constructs: <#2714#>let<#2714#>, <#2715#>block<#2715#>, and <#2716#>tagbody<#2716#>.
  24. These three constructs may be used independently as building blocks
  25. for other types of constructs.
  26.  
  27. <P>
  28. <BR>
  29. <tex2html_image_mark>#defspec2717#
  30. <BR>
  31.  
  32. <P>
  33. <BR>
  34. <tex2html_image_mark>#defmac2770#
  35. <BR>
  36.  
  37. <P>
  38. <BR>
  39. <tex2html_image_mark>#defspec2863#
  40. <BR>
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  42. <P>
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