A programming language should be designed so that it will run on the full range of machines in common use. That is, it must only assume capabilities that are available on all commonly-used machines. This allows applications written in the language to be used by the entire community. Since home machines are now standard and laptops are rapidly becoming standard, portability requires that code also run on them.
When standard high-level programming languages were designed, the standard graphics capability was vt100 emulation. A reasonable standard for the 1990's is an 8-bit color-mapped display, resolution at least 640x480, running the X-window system (defined as Xlib and a window manager) or a window system with similar capabilities. Envision assumes these capabilities, but only these capabilities. (Users of high-end machines may, of course, extend the language to take advantage of their hardware.)
We assume that a mouse will normally be available. However, it is possible to be without a mouse (particularly on a laptop and on new linux installations). Mouse handlers can become wedged, both at the operating system level and in X windows. And, for a touch typist, removing one's hands from the keyboard dramatically slows input. Therefore, we provide keystroke alternatives whenever possible.
Scheme48 and Envision run under Unix, i.e. on workstations and on DOS machines running Linux or FreeBSD. Nothing would, in principle, prevent the language from being implemented on other operating systems. However, we don't plan to do this ourselves. We assume that the CPU is at least as fast as a 66MHz 486 and there is at least 16M of RAM. (Naturally, the system will run much faster on more powerful machines.)
While there are some non-graphical terminals, non-color displays, and sub-486 machines still in use, we believe they will be replaced in the very near future.
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