feat: Preview can now show presets and validate them (#149)

This PR provides a mechanism to "lint" presets to ensure that they refer
to parameters that actually exist and define values that are actually
valid for those parameters.

Relates to:  coder/coder#17333

<img width="486" alt="Screenshot 2025-06-18 at 12 03 37"
src="https://github.com/user-attachments/assets/0b95dd3a-25d4-427a-9905-8ebced5485fd"
/>

Author: SasSwart

cli/clidisplay/resources.go

@@ -74,6 +74,35 @@ func Parameters(writer io.Writer, params []types.Parameter, files map[string]*hc
 	_, _ = fmt.Fprintln(writer, tableWriter.Render())
 }
 
+func Presets(writer io.Writer, presets []types.Preset, files map[string]*hcl.File) {
+	tableWriter := table.NewWriter()
+	tableWriter.SetStyle(table.StyleLight)
+	tableWriter.Style().Options.SeparateColumns = false
+	row := table.Row{"Preset"}
+	tableWriter.AppendHeader(row)
+	for _, p := range presets {
+		tableWriter.AppendRow(table.Row{
+			fmt.Sprintf("%s\n%s", p.Name, formatPresetParameters(p.Parameters)),
+		})
+		if hcl.Diagnostics(p.Diagnostics).HasErrors() {
+			var out bytes.Buffer
+			WriteDiagnostics(&out, files, hcl.Diagnostics(p.Diagnostics))
+			tableWriter.AppendRow(table.Row{out.String()})
+		}
+
+		tableWriter.AppendSeparator()
+	}
+	_, _ = fmt.Fprintln(writer, tableWriter.Render())
+}
+
+func formatPresetParameters(presetParameters map[string]string) string {
+	var str strings.Builder
+	for presetParamName, PresetParamValue := range presetParameters {
+		_, _ = str.WriteString(fmt.Sprintf("%s = %s\n", presetParamName, PresetParamValue))
+	}
+	return str.String()
+}
+
 func formatOptions(selected []string, options []*types.ParameterOption) string {
 	var str strings.Builder
 	sep := ""

cli/root.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"slices"
 	"strings"
 
 	"github.com/hashicorp/hcl/v2"
@@ -27,6 +28,7 @@ func (r *RootCmd) Root() *serpent.Command {
 		vars     []string
 		groups   []string
 		planJSON string
+		preset   string
 	)
 	cmd := &serpent.Command{
 		Use:   "codertf",
@@ -64,10 +66,26 @@ func (r *RootCmd) Root() *serpent.Command {
 				Default:       "",
 				Value:         serpent.StringArrayOf(&groups),
 			},
+			{
+				Name:          "preset",
+				Description:   "Name of the preset to define parameters. Run preview without this flag first to see a list of presets.",
+				Flag:          "preset",
+				FlagShorthand: "s",
+				Default:       "",
+				Value:         serpent.StringOf(&preset),
+			},
 		},
 		Handler: func(i *serpent.Invocation) error {
 			dfs := os.DirFS(dir)
 
+			ctx := i.Context()
+
+			output, _ := preview.Preview(ctx, preview.Input{}, dfs)
+			presets := output.Presets
+			chosenPresetIndex := slices.IndexFunc(presets, func(p types.Preset) bool {
+				return p.Name == preset
+			})
+
 			rvars := make(map[string]string)
 			for _, val := range vars {
 				parts := strings.Split(val, "=")
@@ -76,6 +94,11 @@ func (r *RootCmd) Root() *serpent.Command {
 				}
 				rvars[parts[0]] = parts[1]
 			}
+			if chosenPresetIndex != -1 {
+				for paramName, paramValue := range presets[chosenPresetIndex].Parameters {
+					rvars[paramName] = paramValue
+				}
+			}
 
 			input := preview.Input{
 				PlanJSONPath:    planJSON,
@@ -85,7 +108,6 @@ func (r *RootCmd) Root() *serpent.Command {
 				},
 			}
 
-			ctx := i.Context()
 			output, diags := preview.Preview(ctx, input, dfs)
 			if output == nil {
 				return diags
@@ -103,6 +125,10 @@ func (r *RootCmd) Root() *serpent.Command {
 				clidisplay.WriteDiagnostics(os.Stderr, output.Files, diags)
 			}
 
+			if chosenPresetIndex == -1 {
+				clidisplay.Presets(os.Stdout, presets, output.Files)
+			}
+
 			clidisplay.Parameters(os.Stdout, output.Parameters, output.Files)
 
 			if !output.ModuleOutput.IsNull() && !(output.ModuleOutput.Type().IsObjectType() && output.ModuleOutput.LengthInt() == 0) {

extract/parameter.go

@@ -274,15 +274,18 @@ func requiredString(block *terraform.Block, key string) (string, *hcl.Diagnostic
 		}
 
 		diag := &hcl.Diagnostic{
-			Severity: hcl.DiagError,
-			Summary:  fmt.Sprintf("Invalid %q attribute for block %s", key, block.Label()),
-			Detail:   fmt.Sprintf("Expected a string, got %q", typeName),
-			Subject:  &(tyAttr.HCLAttribute().Range),
-			//Context:     &(block.HCLBlock().DefRange),
-			Expression:  tyAttr.HCLAttribute().Expr,
+			Severity:    hcl.DiagError,
+			Summary:     fmt.Sprintf("Invalid %q attribute for block %s", key, block.Label()),
+			Detail:      fmt.Sprintf("Expected a string, got %q", typeName),
 			EvalContext: block.Context().Inner(),
 		}
 
+		if tyAttr.IsNotNil() {
+			diag.Subject = &(tyAttr.HCLAttribute().Range)
+			// diag.Context = &(block.HCLBlock().DefRange)
+			diag.Expression = tyAttr.HCLAttribute().Expr
+		}
+
 		if !tyVal.IsWhollyKnown() {
 			refs := hclext.ReferenceNames(tyAttr.HCLAttribute().Expr)
 			if len(refs) > 0 {

extract/preset.go

@@ -0,0 +1,45 @@
+package extract
+
+import (
+	"github.com/aquasecurity/trivy/pkg/iac/terraform"
+	"github.com/coder/preview/types"
+	"github.com/hashicorp/hcl/v2"
+)
+
+func PresetFromBlock(block *terraform.Block) types.Preset {
+	p := types.Preset{
+		PresetData: types.PresetData{
+			Parameters: make(map[string]string),
+		},
+		Diagnostics: types.Diagnostics{},
+	}
+
+	if !block.IsResourceType(types.BlockTypePreset) {
+		p.Diagnostics = append(p.Diagnostics, &hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Invalid Preset",
+			Detail:   "Block is not a preset",
+		})
+		return p
+	}
+
+	pName, nameDiag := requiredString(block, "name")
+	if nameDiag != nil {
+		p.Diagnostics = append(p.Diagnostics, nameDiag)
+	}
+	p.Name = pName
+
+	// GetAttribute and AsMapValue both gracefully handle `nil`, `null` and `unknown` values.
+	// All of these return an empty map, which then makes the loop below a no-op.
+	params := block.GetAttribute("parameters").AsMapValue()
+	for presetParamName, presetParamValue := range params.Value() {
+		p.Parameters[presetParamName] = presetParamValue
+	}
+
+	defaultAttr := block.GetAttribute("default")
+	if defaultAttr != nil {
+		p.Default = defaultAttr.Value().True()
+	}
+
+	return p
+}

preset.go

@@ -0,0 +1,58 @@
+package preview
+
+import (
+	"fmt"
+	"slices"
+
+	"github.com/aquasecurity/trivy/pkg/iac/terraform"
+	"github.com/hashicorp/hcl/v2"
+
+	"github.com/coder/preview/extract"
+	"github.com/coder/preview/types"
+)
+
+// presets extracts all presets from the given modules. It then validates the name,
+// parameters and default preset.
+func presets(modules terraform.Modules, parameters []types.Parameter) []types.Preset {
+	foundPresets := make([]types.Preset, 0)
+	var defaultPreset *types.Preset
+
+	for _, mod := range modules {
+		blocks := mod.GetDatasByType(types.BlockTypePreset)
+		for _, block := range blocks {
+			preset := extract.PresetFromBlock(block)
+			switch true {
+			case defaultPreset != nil && preset.Default:
+				preset.Diagnostics = append(preset.Diagnostics, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Multiple default presets",
+					Detail:   fmt.Sprintf("Only one preset can be marked as default. %q is already marked as default", defaultPreset.Name),
+				})
+			case defaultPreset == nil && preset.Default:
+				defaultPreset = &preset
+			}
+
+			for paramName, paramValue := range preset.Parameters {
+				templateParamIndex := slices.IndexFunc(parameters, func(p types.Parameter) bool {
+					return p.Name == paramName
+				})
+				if templateParamIndex == -1 {
+					preset.Diagnostics = append(preset.Diagnostics, &hcl.Diagnostic{
+						Severity: hcl.DiagError,
+						Summary:  "Undefined Parameter",
+						Detail:   fmt.Sprintf("Preset parameter %q is not defined by the template.", paramName),
+					})
+					continue
+				}
+				templateParam := parameters[templateParamIndex]
+				for _, diag := range templateParam.Valid(types.StringLiteral(paramValue)) {
+					preset.Diagnostics = append(preset.Diagnostics, diag)
+				}
+			}
+
+			foundPresets = append(foundPresets, preset)
+		}
+	}
+
+	return foundPresets
+}

preview.go

@@ -41,6 +41,7 @@ type Output struct {
 
 	Parameters    []types.Parameter `json:"parameters"`
 	WorkspaceTags types.TagBlocks   `json:"workspace_tags"`
+	Presets       []types.Preset    `json:"presets"`
 	// Files is included for printing diagnostics.
 	// They can be marshalled, but not unmarshalled. This is a limitation
 	// of the HCL library.
@@ -178,6 +179,7 @@ func Preview(ctx context.Context, input Input, dir fs.FS) (output *Output, diagn
 
 	diags := make(hcl.Diagnostics, 0)
 	rp, rpDiags := parameters(modules)
+	presets := presets(modules, rp)
 	tags, tagDiags := workspaceTags(modules, p.Files())
 
 	// Add warnings
@@ -187,6 +189,7 @@ func Preview(ctx context.Context, input Input, dir fs.FS) (output *Output, diagn
 		ModuleOutput:  outputs,
 		Parameters:    rp,
 		WorkspaceTags: tags,
+		Presets:       presets,
 		Files:         p.Files(),
 	}, diags.Extend(rpDiags).Extend(tagDiags)
 }

preview_test.go

@@ -43,6 +43,7 @@ func Test_Extract(t *testing.T) {
 		expTags     map[string]string
 		unknownTags []string
 		params      map[string]assertParam
+		presets     func(t *testing.T, presets []types.Preset)
 		warnings    []*regexp.Regexp
 	}{
 		{
@@ -243,6 +244,62 @@ func Test_Extract(t *testing.T) {
 					errorDiagnostics("Required"),
 			},
 		},
+		{
+			name:        "invalid presets",
+			dir:         "invalidpresets",
+			expTags:     map[string]string{},
+			input:       preview.Input{},
+			unknownTags: []string{},
+			params: map[string]assertParam{
+				"valid_parameter_name": ap().
+					optVals("valid_option_value"),
+			},
+			presets: func(t *testing.T, presets []types.Preset) {
+				presetMap := map[string]func(t *testing.T, preset types.Preset){
+					"empty_parameters": func(t *testing.T, preset types.Preset) {
+						require.Len(t, preset.Diagnostics, 0)
+					},
+					"no_parameters": func(t *testing.T, preset types.Preset) {
+						require.Len(t, preset.Diagnostics, 0)
+					},
+					"invalid_parameter_name": func(t *testing.T, preset types.Preset) {
+						require.Len(t, preset.Diagnostics, 1)
+						require.Equal(t, preset.Diagnostics[0].Summary, "Undefined Parameter")
+						require.Equal(t, preset.Diagnostics[0].Detail, "Preset parameter \"invalid_parameter_name\" is not defined by the template.")
+					},
+					"invalid_parameter_value": func(t *testing.T, preset types.Preset) {
+						require.Len(t, preset.Diagnostics, 1)
+						require.Equal(t, preset.Diagnostics[0].Summary, "Value must be a valid option")
+						require.Equal(t, preset.Diagnostics[0].Detail, "the value \"invalid_value\" must be defined as one of options")
+					},
+					"valid_preset": func(t *testing.T, preset types.Preset) {
+						require.Len(t, preset.Diagnostics, 0)
+						require.Equal(t, preset.Parameters, map[string]string{
+							"valid_parameter_name": "valid_option_value",
+						})
+					},
+				}
+
+				for _, preset := range presets {
+					if fn, ok := presetMap[preset.Name]; ok {
+						fn(t, preset)
+					}
+				}
+
+				var defaultPresetsWithError int
+				for _, preset := range presets {
+					if preset.Name == "default_preset" || preset.Name == "another_default_preset" {
+						for _, diag := range preset.Diagnostics {
+							if diag.Summary == "Multiple default presets" {
+								defaultPresetsWithError++
+								break
+							}
+						}
+					}
+				}
+				require.Equal(t, 1, defaultPresetsWithError, "exactly one default preset should have the multiple defaults error")
+			},
+		},
 		{
 			name:        "required",
 			dir:         "required",
@@ -575,6 +632,11 @@ func Test_Extract(t *testing.T) {
 				require.True(t, ok, "unknown parameter %s", param.Name)
 				check(t, param)
 			}
+
+			// Assert presets
+			if tc.presets != nil {
+				tc.presets(t, output.Presets)
+			}
 		})
 	}
 }