13.3 Interactive File Control

If input is coming from an external file, the system treats it as a batch processed calculation. If the user desires interactive response in this case, he can include the command ON INT; in the file. Likewise, he can issue the command off int; in the main program if he does not desire continual questioning from the system. Regardless of the setting of INT, input commands from a file are not kept in the system, and so cannot be edited using ED. However, many implementations of REDUCE provide a link to an external system editor that can be used for such editing. The specific instructions for the particular implementation should be consulted for information on this.

Two commands are available in REDUCE for interactive use of files. PAUSE; may be inserted at any point in an input file. When this command is encountered on input, the system prints the message CONT? on the user’s terminal and halts. If the user responds Y (for yes), the calculation continues from that point in the file. If the user responds N (for no), control is returned to the terminal, and the user can input further statements and commands. Later on he can use the command cont; to transfer control back to the point in the file following the last PAUSE encountered. A top-level pause; from the user’s terminal has no effect.