Add --system-prompt and --system-prompt-file flags

These allow to set a different system prompt for conversations and
one-shot prompts.

Also add a new `modelDefaults.systemPrompt` configuration key to define
the default system prompt, which can be overriden per-execution with the
--system-prompt or --system-prompt-file flags.

pkg/cli/cmd.go

@@ -12,6 +12,8 @@ import (
 var (
 	maxTokens        int
 	model            string
+	systemPrompt     string
+	systemPromptFile string
 )
 
 func init() {
@@ -19,6 +21,9 @@ func init() {
 	for _, cmd := range inputCmds {
 		cmd.Flags().IntVar(&maxTokens, "length", *config.OpenAI.DefaultMaxLength, "Max response length in tokens")
 		cmd.Flags().StringVar(&model, "model", *config.OpenAI.DefaultModel, "The language model to use")
+		cmd.Flags().StringVar(&systemPrompt, "system-prompt", *config.ModelDefaults.SystemPrompt, "The system prompt to use.")
+		cmd.Flags().StringVar(&systemPromptFile, "system-prompt-file", "", "A path to a file whose contents are used as the system prompt.")
+		cmd.MarkFlagsMutuallyExclusive("system-prompt", "system-prompt-file")
 	}
 
 	rootCmd.AddCommand(
@@ -35,6 +40,17 @@ func Execute() error {
 	return rootCmd.Execute()
 }
 
+func SystemPrompt() string {
+	if systemPromptFile != "" {
+		content, err := FileContents(systemPromptFile)
+		if err != nil {
+			Fatal("Could not read file contents at %s: %v", systemPromptFile, err)
+		}
+		return content
+	}
+	return systemPrompt
+}
+
 // InputFromArgsOrEditor returns either the provided input from the args slice
 // (joined with spaces), or if len(args) is 0, opens an editor and returns
 // whatever input was provided there. placeholder is a string which populates
@@ -327,12 +343,11 @@ var newCmd = &cobra.Command{
 			Fatal("Could not save new conversation: %v\n", err)
 		}
 
-		const system = "You are a helpful assistant."
 		messages := []Message{
 			{
 				ConversationID:  conversation.ID,
 				Role:            "system",
-				OriginalContent: system,
+				OriginalContent: SystemPrompt(),
 			},
 			{
 				ConversationID:  conversation.ID,
@@ -396,11 +411,10 @@ var promptCmd = &cobra.Command{
 			Fatal("No message was provided.\n")
 		}
 
-		const system = "You are a helpful assistant."
 		messages := []Message{
 			{
 				Role:            "system",
-				OriginalContent: system,
+				OriginalContent: SystemPrompt(),
 			},
 			{
 				Role:            "user",

pkg/cli/config.go

@@ -9,6 +9,9 @@ import (
 )
 
 type Config struct {
+	ModelDefaults *struct {
+		SystemPrompt *string `yaml:"systemPrompt" default:"You are a helpful assistant."`
+	} `yaml:"modelDefaults"`
 	OpenAI *struct {
 		APIKey           *string `yaml:"apiKey" default:"your_key_here"`
 		DefaultModel     *string `yaml:"defaultModel" default:"gpt-4"`

pkg/cli/util.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"reflect"
 	"strconv"
 	"strings"
@@ -158,3 +159,15 @@ func SetStructDefaults(data interface{}) bool {
 	}
 	return changed
 }
+
+// FileContents returns the string contents of the given file.
+// TODO: we should support retrieving the content (or an approximation of)
+// non-text documents, e.g. PDFs.
+func FileContents(file string) (string, error) {
+	path := filepath.Clean(file)
+	content, err := os.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+	return strings.Trim(string(content), "\n\t "), nil
+}