cmd/hcldec: allow spec file to define variables and functions

The spec file can now additionally define default variables and functions
for the eval context used to evaluate the input file.

3 files changed, 140 insertions, 11 deletions

diff --git a/cmd/hcldec/main.go b/cmd/hcldec/main.go
index 9ace52d..6bb3bf7 100644
--- a/cmd/hcldec/main.go
+++ b/cmd/hcldec/main.go
@@ -12,6 +12,7 @@ import (
 	"github.com/hashicorp/hcl2/hclparse"
 	flag "github.com/spf13/pflag"
 	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/function"
 	ctyjson "github.com/zclconf/go-cty/cty/json"
 	"golang.org/x/crypto/ssh/terminal"
 )
@@ -69,18 +70,26 @@ func realmain(args []string) error {
 
 	var diags hcl.Diagnostics
 
-	spec, specDiags := loadSpecFile(*specFile)
+	specContent, specDiags := loadSpecFile(*specFile)
 	diags = append(diags, specDiags...)
 	if specDiags.HasErrors() {
 		diagWr.WriteDiagnostics(diags)
 		os.Exit(2)
 	}
 
-	var ctx *hcl.EvalContext
+	spec := specContent.RootSpec
+
+	ctx := &hcl.EvalContext{
+		Variables: map[string]cty.Value{},
+		Functions: map[string]function.Function{},
+	}
+	for name, val := range specContent.Variables {
+		ctx.Variables[name] = val
+	}
+	for name, f := range specContent.Functions {
+		ctx.Functions[name] = f
+	}
 	if len(*vars) != 0 {
-		ctx = &hcl.EvalContext{
-			Variables: map[string]cty.Value{},
-		}
 		for i, varsSpec := range *vars {
 			var vals map[string]cty.Value
 			var valsDiags hcl.Diagnostics
@@ -98,6 +107,19 @@ func realmain(args []string) error {
 		}
 	}
 
+	// If we have empty context elements then we'll nil them out so that
+	// we'll produce e.g. "variables are not allowed" errors instead of
+	// "variable not found" errors.
+	if len(ctx.Variables) == 0 {
+		ctx.Variables = nil
+	}
+	if len(ctx.Functions) == 0 {
+		ctx.Functions = nil
+	}
+	if ctx.Variables == nil && ctx.Functions == nil {
+		ctx = nil
+	}
+
 	var bodies []hcl.Body
 
 	if len(args) == 0 {
diff --git a/cmd/hcldec/spec-format.md b/cmd/hcldec/spec-format.md
index 69b21b8..d80beda 100644
--- a/cmd/hcldec/spec-format.md
+++ b/cmd/hcldec/spec-format.md
@@ -275,7 +275,7 @@ literal {
 `literal` spec blocks accept the following argument:
 
 * `value` (required) - The value to return. This attribute may be an expression
-  that uses [functions](#functions).
+  that uses [functions](#spec-definition-functions).
 
 `literal` is a leaf spec type, so no nested spec blocks are permitted.
 
@@ -331,9 +331,62 @@ transform {
   spec. The variable `nested` is defined when evaluating this expression, with
   the result value of the nested spec.
 
-The `result` expression may use [functions](#functions).
+The `result` expression may use [functions](#spec-definition-functions).
 
-## Functions
+## Predefined Variables
+
+`hcldec` accepts values for variables to expose into the input file's
+expression scope as CLI options, and this is the most common way to pass
+values since it allows them to be dynamically populated by the calling
+application.
+
+However, it's also possible to pre-define variables with constant values
+within a spec file, using the top-level `variables` block type:
+
+```hcl
+variables {
+  name = "Stephen"
+}
+```
+
+Variables of the same name defined via the `hcldec` command line with override
+predefined variables of the same name, so this mechanism can also be used to
+provide defaults for variables that are overridden only in certain contexts.
+
+## Custom Functions
+
+The spec can make arbitrary HCL functions available in the input file's
+expression scope, and thus allow simple computation within the input file,
+in addition to HCL's built-in operators.
+
+Custom functions are defined in the spec file with the top-level `function`
+block type:
+
+```
+function "add_one" {
+  params = ["n"]
+  result = n + 1
+}
+```
+
+Functions behave in a similar way to the `transform` spec type in that the
+given `result` attribute expression is evaluated with additional variables
+defined with the same names as the defined `params`.
+
+The [spec definition functions](#spec-definition-functions) can be used within
+custom function expressions, allowing them to be optionally exposed into the
+input file:
+
+```
+function "upper" {
+  params = ["str"]
+  result = upper(str)
+}
+```
+
+Custom functions defined in the spec cannot be called from the spec itself.
+
+## Spec Definition Functions
 
 Certain expressions within a specification may use the following functions.
 The documentation for each spec type above specifies where functions may
@@ -355,6 +408,11 @@ be used.
 * `substr(string, offset, length)` returns the requested substring of the given string.
 * `upper(string)` returns the given string with all lowercase letters converted to uppercase.
 
+Note that these expressions are valid in the context of the _spec_ file, not
+the _input_. Functions can be exposed into the input file using
+[Custom Functions](#custom-functions) within the spec, which may in turn
+refer to these spec definition functions.
+
 ## Type Expressions
 
 Type expressions are used to describe the expected type of an attribute, as
diff --git a/cmd/hcldec/spec.go b/cmd/hcldec/spec.go
index 8010919..31effd3 100644
--- a/cmd/hcldec/spec.go
+++ b/cmd/hcldec/spec.go
@@ -3,23 +3,72 @@ package main
 import (
 	"fmt"
 
+	"github.com/hashicorp/hcl2/ext/userfunc"
 	"github.com/hashicorp/hcl2/gohcl"
 	"github.com/hashicorp/hcl2/hcl"
 	"github.com/hashicorp/hcl2/hcldec"
 	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/function"
 )
 
+type specFileContent struct {
+	Variables map[string]cty.Value
+	Functions map[string]function.Function
+	RootSpec  hcldec.Spec
+}
+
 var specCtx = &hcl.EvalContext{
 	Functions: specFuncs,
 }
 
-func loadSpecFile(filename string) (hcldec.Spec, hcl.Diagnostics) {
+func loadSpecFile(filename string) (specFileContent, hcl.Diagnostics) {
 	file, diags := parser.ParseHCLFile(filename)
 	if diags.HasErrors() {
-		return errSpec, diags
+		return specFileContent{RootSpec: errSpec}, diags
+	}
+
+	vars, funcs, specBody, declDiags := decodeSpecDecls(file.Body)
+	diags = append(diags, declDiags...)
+
+	spec, specDiags := decodeSpecRoot(specBody)
+	diags = append(diags, specDiags...)
+
+	return specFileContent{
+		Variables: vars,
+		Functions: funcs,
+		RootSpec:  spec,
+	}, diags
+}
+
+func decodeSpecDecls(body hcl.Body) (map[string]cty.Value, map[string]function.Function, hcl.Body, hcl.Diagnostics) {
+	funcs, body, diags := userfunc.DecodeUserFunctions(body, "function", func() *hcl.EvalContext {
+		return specCtx
+	})
+
+	content, body, moreDiags := body.PartialContent(&hcl.BodySchema{
+		Blocks: []hcl.BlockHeaderSchema{
+			{
+				Type: "variables",
+			},
+		},
+	})
+	diags = append(diags, moreDiags...)
+
+	vars := make(map[string]cty.Value)
+	for _, block := range content.Blocks {
+		// We only have one block type in our schema, so we can assume all
+		// blocks are of that type.
+		attrs, moreDiags := block.Body.JustAttributes()
+		diags = append(diags, moreDiags...)
+
+		for name, attr := range attrs {
+			val, moreDiags := attr.Expr.Value(specCtx)
+			diags = append(diags, moreDiags...)
+			vars[name] = val
+		}
 	}
 
-	return decodeSpecRoot(file.Body)
+	return vars, funcs, body, diags
 }
 
 func decodeSpecRoot(body hcl.Body) (hcldec.Spec, hcl.Diagnostics) {