Output

Dyalog produces output through several distinct mechanisms. This section defines the terminology for these mechanisms and describes where each type of output goes, both in the interactive Session and when the interpreter is attached to operating-system streams.

Types of Output

Implicit output is the display of the result of an expression that is not assigned to a name, passed as an argument to a function or operator, or otherwise suppressed. This is also known as "default output", "direct output", or "numeric output". It arises at the Session prompt and from unassigned values in traditional functions and operators.

Output is also produced explicitly by assignment to ⎕, which displays an array in the same form as implicit output, or to ⍞, which displays characters without a trailing new-line. Error messages, the output of system commands, and the messages reported by the function editor are all directed in the same way as output through ⍞.

Output in the Interactive Session

In the interactive Session, whether the native GUI or a Ride connection, all output appears in the Session log; the standard output (stdout) and standard error (stderr) streams described below do not apply.

Implicit output and output through ⎕ are wrapped according to the print width ⎕PW. Output through ⍞ ignores ⎕PW and instead wraps at the width of the window or terminal.

The Session reports a SessionPrint event immediately before a value is displayed, irrespective of whether that value is implicit output or output through ⎕. A callback attached to this event takes over the display entirely: it might output anything, or nothing, by any means (for example, through ⎕, ⍞, or otherwise). The event is not reported for error messages or for the output of system commands.

Output to Operating-System Streams

When the interpreter is attached to operating-system streams, running as a shell script, or started from a terminal with its streams redirected, each type of output is written to either standard output (stdout) or standard error (stderr) as identified in Table 1:

Example

The following script redirects the two streams to separate destinations, sending each message to its own destination.

#!/usr/local/bin/dyalogscript
⎕←'to stdout'
⍞←'to stderr'

Print Width and Streams

⎕PW sets the width at which output is wrapped. It applies only to implicit output and to output through ⎕; it does not affect output through ⍞, nor the result of format (⍕), ⎕FMT, ⎕ARBOUT, or ⎕ARBIN. Because ⍞ is the only value-bearing output written to stderr, ⎕PW governs stdout alone.