Refactor variables: Keep order of declaration

This shouldn't have any behavior changes for now. This is a code
refactor that should allow us to do further improvements on how
variables are handled, specially regarding respecting the declaration
order in Taskfiles, which should make it easier for the users.

Initial work on #218

internal/taskfile/var.go

@@ -3,6 +3,8 @@ package taskfile
 import (
 	"errors"
 	"strings"
+
+	"gopkg.in/yaml.v3"
 )
 
 var (
@@ -11,17 +13,86 @@ var (
 )
 
 // Vars is a string[string] variables map.
-type Vars map[string]Var
+type Vars struct {
+	Keys    []string
+	Mapping map[string]Var
+}
+
+// UnmarshalYAML implements the yaml.Unmarshaler interface.
+func (vs *Vars) UnmarshalYAML(node *yaml.Node) error {
+	if node.Kind != yaml.MappingNode {
+		return errors.New("task: vars is not a map")
+	}
+
+	// NOTE(@andreynering): on this style of custom unmarsheling,
+	// even number contains the keys, while odd numbers contains
+	// the values.
+	for i := 0; i < len(node.Content); i += 2 {
+		keyNode := node.Content[i]
+		valueNode := node.Content[i+1]
+
+		var v Var
+		if err := valueNode.Decode(&v); err != nil {
+			return err
+		}
+		vs.Set(keyNode.Value, v)
+	}
+	return nil
+}
+
+// Merge merges the given Vars into the caller one
+func (vs *Vars) Merge(other *Vars) {
+	other.Range(func(key string, value Var) error {
+		vs.Set(key, value)
+		return nil
+	})
+}
+
+// Set sets a value to a given key
+func (vs *Vars) Set(key string, value Var) {
+	if vs == nil {
+		vs = &Vars{}
+	}
+	if vs.Mapping == nil {
+		vs.Mapping = make(map[string]Var, 1)
+	}
+	if !strSliceContains(vs.Keys, key) {
+		vs.Keys = append(vs.Keys, key)
+	}
+	vs.Mapping[key] = value
+}
+
+func strSliceContains(s []string, str string) bool {
+	for _, v := range s {
+		if v == str {
+			return true
+		}
+	}
+	return false
+}
+
+// Range allows you to loop into the vars in its right order
+func (vs *Vars) Range(yield func(key string, value Var) error) error {
+	if vs == nil {
+		return nil
+	}
+	for _, k := range vs.Keys {
+		if err := yield(k, vs.Mapping[k]); err != nil {
+			return err
+		}
+	}
+	return nil
+}
 
 // ToCacheMap converts Vars to a map containing only the static
 // variables
-func (vs Vars) ToCacheMap() (m map[string]interface{}) {
-	m = make(map[string]interface{}, len(vs))
-	for k, v := range vs {
+func (vs *Vars) ToCacheMap() (m map[string]interface{}) {
+	m = make(map[string]interface{}, len(vs.Keys))
+	vs.Range(func(k string, v Var) error {
 		if v.Sh != "" {
 			// Dynamic variable is not yet resolved; trigger
 			// <no value> to be used in templates.
-			continue
+			return nil
 		}
 
 		if v.Live != nil {
@@ -29,7 +100,8 @@ func (vs Vars) ToCacheMap() (m map[string]interface{}) {
 		} else {
 			m[k] = v.Static
 		}
-	}
+		return nil
+	})
 	return
 }