diff --git a/decoder/signature_test.go b/decoder/signature_test.go
new file mode 100644
index 00000000..6938c47a
--- /dev/null
+++ b/decoder/signature_test.go
@@ -0,0 +1,426 @@
+package decoder
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/hashicorp/hcl-lang/lang"
+	"github.com/hashicorp/hcl-lang/schema"
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/function"
+)
+
+func TestSignatureAtPos(t *testing.T) {
+	testCases := []struct {
+		testName          string
+		functions         map[string]schema.FunctionSignature
+		cfg               string
+		pos               hcl.Pos
+		expectedSignature *lang.FunctionSignature
+	}{
+		{
+			"no function call expr",
+			map[string]schema.FunctionSignature{},
+			`x = "hello"`,
+			hcl.Pos{Line: 1, Column: 3, Byte: 4},
+			nil,
+		},
+		{
+			"unknown function",
+			map[string]schema.FunctionSignature{},
+			`x = foo()`,
+			hcl.Pos{Line: 1, Column: 7, Byte: 8},
+			nil,
+		},
+		{
+			"no description",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					ReturnType: cty.String,
+				},
+			},
+			`x = foo()`,
+			hcl.Pos{Line: 1, Column: 7, Byte: 8},
+			&lang.FunctionSignature{
+				Name:        "foo() string",
+				Description: lang.Markdown(""),
+			},
+		},
+		{
+			"no parameter, no var param",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo()`,
+			hcl.Pos{Line: 1, Column: 7, Byte: 8},
+			&lang.FunctionSignature{
+				Name:        "foo() string",
+				Description: lang.Markdown("`foo` description"),
+			},
+		},
+		{
+			"one parameter, pos not inside parenthesis",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "input",
+							Type:        cty.String,
+							Description: "`input` description",
+						},
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description.",
+				},
+			},
+			`x = foo()`,
+			hcl.Pos{Line: 1, Column: 5, Byte: 6},
+			nil,
+		},
+		{
+			"one parameter, empty first parameter",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "input",
+							Type:        cty.String,
+							Description: "`input` description",
+						},
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo()`,
+			hcl.Pos{Line: 1, Column: 7, Byte: 8},
+			&lang.FunctionSignature{
+				Name:        "foo(input string) string",
+				Description: lang.Markdown("`foo` description"),
+				Parameters: []lang.FunctionParameter{
+					{
+						Name:        "input",
+						Description: lang.Markdown("`input` description"),
+					},
+				},
+				ActiveParameter: 0,
+			},
+		},
+		{
+			"two parameters, empty second parameter",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "input",
+							Type:        cty.String,
+							Description: "`input` description",
+						},
+						{
+							Name:        "input2",
+							Type:        cty.Number,
+							Description: "`input2` description",
+						},
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo("a", )`,
+			hcl.Pos{Line: 1, Column: 12, Byte: 13},
+			&lang.FunctionSignature{
+				Name:        "foo(input string, input2 number) string",
+				Description: lang.Markdown("`foo` description"),
+				Parameters: []lang.FunctionParameter{
+					{
+						Name:        "input",
+						Description: lang.Markdown("`input` description"),
+					},
+					{
+						Name:        "input2",
+						Description: lang.Markdown("`input2` description"),
+					},
+				},
+				ActiveParameter: 1,
+			},
+		},
+		{
+			"one parameter, one variadic, empty variadic",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "input",
+							Type:        cty.String,
+							Description: "`input` description",
+						},
+					},
+					VarParam: &function.Parameter{
+						Name:        "vinput",
+						Type:        cty.Number,
+						Description: "`vinput` description",
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo("a", )`,
+			hcl.Pos{Line: 1, Column: 12, Byte: 13},
+			&lang.FunctionSignature{
+				Name:        "foo(input string, ...vinput number) string",
+				Description: lang.Markdown("`foo` description"),
+				Parameters: []lang.FunctionParameter{
+					{
+						Name:        "input",
+						Description: lang.Markdown("`input` description"),
+					},
+					{
+						Name:        "vinput",
+						Description: lang.Markdown("`vinput` description"),
+					},
+				},
+				ActiveParameter: 1,
+			},
+		},
+		{
+			"one parameter, one variadic, some variadic",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "input",
+							Type:        cty.String,
+							Description: "`input` description",
+						},
+					},
+					VarParam: &function.Parameter{
+						Name:        "vinput",
+						Type:        cty.Number,
+						Description: "`vinput` description",
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo("a", 1, 2, )`,
+			hcl.Pos{Line: 1, Column: 18, Byte: 19},
+			&lang.FunctionSignature{
+				Name:        "foo(input string, ...vinput number) string",
+				Description: lang.Markdown("`foo` description"),
+				Parameters: []lang.FunctionParameter{
+					{
+						Name:        "input",
+						Description: lang.Markdown("`input` description"),
+					},
+					{
+						Name:        "vinput",
+						Description: lang.Markdown("`vinput` description"),
+					},
+				},
+				ActiveParameter: 1,
+			},
+		},
+		{
+			"two complex parameters, empty second parameter",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "input",
+							Type:        cty.List(cty.String),
+							Description: "`input` description",
+						},
+						{
+							Name:        "input2",
+							Type:        cty.List(cty.Number),
+							Description: "`input2` description",
+						},
+					},
+					ReturnType:  cty.Map(cty.Number),
+					Description: "`foo` description",
+				},
+			},
+			`x = foo(["a", "b"], )`,
+			hcl.Pos{Line: 1, Column: 19, Byte: 20},
+			&lang.FunctionSignature{
+				Name:        "foo(input list of string, input2 list of number) map of number",
+				Description: lang.Markdown("`foo` description"),
+				Parameters: []lang.FunctionParameter{
+					{
+						Name:        "input",
+						Description: lang.Markdown("`input` description"),
+					},
+					{
+						Name:        "input2",
+						Description: lang.Markdown("`input2` description"),
+					},
+				},
+				ActiveParameter: 1,
+			},
+		},
+		{
+			"two parameters, named similar to function",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "foo",
+							Type:        cty.String,
+							Description: "`foo` description",
+						},
+						{
+							Name:        "foo2",
+							Type:        cty.Number,
+							Description: "`foo2` description",
+						},
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo()`,
+			hcl.Pos{Line: 1, Column: 7, Byte: 8},
+			&lang.FunctionSignature{
+				Name:        "foo(foo string, foo2 number) string",
+				Description: lang.Markdown("`foo` description"),
+				Parameters: []lang.FunctionParameter{
+					{
+						Name:        "foo",
+						Description: lang.Markdown("`foo` description"),
+					},
+					{
+						Name:        "foo2",
+						Description: lang.Markdown("`foo2` description"),
+					},
+				},
+				ActiveParameter: 0,
+			},
+		},
+		{
+			"three parameters, filling middle parameter",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "input",
+							Type:        cty.String,
+							Description: "`input` description",
+						},
+						{
+							Name:        "input2",
+							Type:        cty.Number,
+							Description: "`input2` description",
+						},
+						{
+							Name:        "input3",
+							Type:        cty.Bool,
+							Description: "`input3` description",
+						},
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo("a",  , false)`,
+			hcl.Pos{Line: 1, Column: 12, Byte: 13},
+			&lang.FunctionSignature{
+				Name:        "foo(input string, input2 number, input3 bool) string",
+				Description: lang.Markdown("`foo` description"),
+				Parameters: []lang.FunctionParameter{
+					{
+						Name:        "input",
+						Description: lang.Markdown("`input` description"),
+					},
+					{
+						Name:        "input2",
+						Description: lang.Markdown("`input2` description"),
+					},
+					{
+						Name:        "input3",
+						Description: lang.Markdown("`input3` description"),
+					},
+				},
+				ActiveParameter: 1,
+			},
+		},
+		{
+			"two parameters, adding a third",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					Params: []function.Parameter{
+						{
+							Name:        "foo",
+							Type:        cty.String,
+							Description: "`foo` description",
+						},
+						{
+							Name:        "foo2",
+							Type:        cty.Number,
+							Description: "`foo2` description",
+						},
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo("a", "b", )`,
+			hcl.Pos{Line: 1, Column: 17, Byte: 18},
+			nil,
+		},
+		{
+			"no parameter, one variadic, some variadic",
+			map[string]schema.FunctionSignature{
+				"foo": {
+					VarParam: &function.Parameter{
+						Name:        "vinput",
+						Type:        cty.Number,
+						Description: "`vinput` description",
+					},
+					ReturnType:  cty.String,
+					Description: "`foo` description",
+				},
+			},
+			`x = foo(1, 2, 3, )`,
+			hcl.Pos{Line: 1, Column: 16, Byte: 17},
+			&lang.FunctionSignature{
+				Name:        "foo(...vinput number) string",
+				Description: lang.Markdown("`foo` description"),
+				Parameters: []lang.FunctionParameter{
+					{
+						Name:        "vinput",
+						Description: lang.Markdown("`vinput` description"),
+					},
+				},
+				ActiveParameter: 0,
+			},
+		},
+	}
+
+	for i, tc := range testCases {
+		t.Run(fmt.Sprintf("%2d-%s", i, tc.testName), func(t *testing.T) {
+			f, _ := hclsyntax.ParseConfig([]byte(tc.cfg), "test.tf", hcl.InitialPos)
+			d := testPathDecoder(t, &PathContext{
+				Files: map[string]*hcl.File{
+					"test.tf": f,
+				},
+				Functions: tc.functions,
+			})
+
+			signature, err := d.SignatureAtPos("test.tf", tc.pos)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if diff := cmp.Diff(tc.expectedSignature, signature); diff != "" {
+				t.Fatalf("unexpected signature: %s", diff)
+			}
+		})
+	}
+
+}