Streams
Streams in Shen were introduced to encode some of the functions that were hard-wired into Qi II like 'write-file' and 'read-file'. The goal was to capture just as much in the way of primitives as is necessary to reproduce the functionality of Qi II. From Shen 13 onwards only byte streams are supported by the standard. Any other kind of stream will be found in a library. See the side link
Bytes and Characters' for more on reading and writing to Shen streams.
The basic functions for streams are
- open
- close
- pr
- read-byte
- write-byte
- stinput
- stoutput
The function 'open' creates a stream and the first argument is a string designating the file which is the basis
of the stream and the second argument is either 'in' or 'out' determines whether the resulting stream is a source
or sink . Material written to a file overwrites the contents already in it. The function 'close' closes a stream
and returns the empty list.
The 'open' function works in hand with the 'cd' command in Shen which fixes the home directory. All files are opened
relative to the value for the home directory, which is held in the global *home-directory*. The value of this
global is changed by the cd function common to Qi and Shen.
(stinput) returns the standard input as a byte source. If 'read-byte' is applied to the standard input then the
user's input is echoed to the terminal under the printing convention that the character printed has the same
code point as the byte (as in UTF-8). Effectively this means that (stinput) may handle extended ASCII, although
only ASCII is required of the Shen reader.
'write-byte' writes a byte as a number n from 0 to 255 to a stream and returns n as a value. If the stream is
omitted then 'write-byte' defaults to the standard output. If 'write-byte' is applied to the standard output then
the user's input is echoed to the terminal under the printing convention that the character printed has the
same code point as the byte. (stoutput) returns the standard output.
Both 'stinput' and 'stoutput' are defined in Shen within the Shen kernel in terms of the global variables
*stinput* and *stoutput* to which the Shen standard input and the Shen standard output are bound.
The type 'stream' is a parametric dependent type in Shen and has only two possible parameters - 'in' or 'out'. The type of
the 'open' function is a dependent type which cannot be given a type in arrow notation but requires the following
sequent calculus definition.
if (element? Direction [in out])
File : string;
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(open File Direction) : (stream Direction);
where the values of Direction are 'in' and 'out'.
In Shen 16 the zero-place function 'it' was introduced; (it) returns the last (unevaluated) expression entered to
the standard input as a string. The type of it is (--> string). |
