An '''Application Program(ming) Interface''', or '''API''', is a mechanism by
which one software system or component may interact with another.  [Tcl] and
[Tk] both have [script]-level and [C]-level ([Tcl C API]/[Tk C API]) APIs.



** Description **

When designing an API, consider '''exposing functionality rather than exposing
data'''.   When clients of an [API] access data through
commands/functions/procedures instead of directly, the API provides a layer of
abstraction between the clients and the code base.  When clients access data
directly, there's an additional burden on developers of the code base not to
break said clients.  Interfaces that do expose data are often eventually
changed to expose functionality instead.  This has happened with the the Tcl
[C] [API].  Examples include [Tcl_SetObjResult] and [TIP]
[http://www.tcl.tk/cgi-bin/tct/tip/445.html%|%445, Tcl_ObjType Utility
Routines].



** Further Reading **

   [http://www.itworld.com/article/2786353/development/apis-considered-harmful.html%|%APIs Considered Harmful], Sean McGrath, 2002-04-18:   A critique of meta APIs such as those that try to abstract services available over [HTTP].

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