diff --git a/src/commands/macros.rs b/src/commands/macros.rs
index bb5e96d0..5a47ffe0 100644
--- a/src/commands/macros.rs
+++ b/src/commands/macros.rs
@@ -262,7 +262,9 @@ macro_rules! command {
             Extract {
                 $($extract:tt)* {
                     use std::convert::TryInto;
-                    $args.expect_nth($($positional_count)*)?.try_into()?
+                    let value = $args.expect_nth($($positional_count)*)?;
+                    let value = $param_kind.check(value)?;
+                    value.extract()
                 }
             }
         );
diff --git a/src/commands/open.rs b/src/commands/open.rs
index a37f5f37..34a6490d 100644
--- a/src/commands/open.rs
+++ b/src/commands/open.rs
@@ -1,7 +1,6 @@
 use crate::errors::ShellError;
 use crate::object::{Primitive, Switch, Value};
 use crate::parser::parse::span::Span;
-use crate::parser::registry::NamedType;
 use crate::prelude::*;
 use mime::Mime;
 use std::path::{Path, PathBuf};
diff --git a/src/commands/where_.rs b/src/commands/where_.rs
index dca4816a..9ea08b4f 100644
--- a/src/commands/where_.rs
+++ b/src/commands/where_.rs
@@ -1,5 +1,5 @@
 use crate::errors::ShellError;
-use crate::object::Block;
+use crate::object::types::*;
 use crate::prelude::*;
 use futures::future::ready;
 use log::trace;
diff --git a/src/object/base.rs b/src/object/base.rs
index 1125cc5a..54693314 100644
--- a/src/object/base.rs
+++ b/src/object/base.rs
@@ -149,11 +149,6 @@ impl Deserialize<'de> for Block {
         D: Deserializer<'de>,
     {
         unimplemented!("deserialize block")
-        // let s = "\"unimplemented deserialize block\"";
-        // Ok(Block::new(
-        //     TokenTreeBuilder::spanned_string((1, s.len() - 1), (0, s.len())),
-        //     Text::from(s),
-        // ))
     }
 }
 
diff --git a/src/object/types.rs b/src/object/types.rs
index 34055e0b..3d026772 100644
--- a/src/object/types.rs
+++ b/src/object/types.rs
@@ -1,7 +1,86 @@
+use crate::object::base as value;
+use crate::parser::hir;
+use crate::prelude::*;
 use derive_new::new;
 use serde_derive::{Deserialize, Serialize};
 
-#[derive(Debug, Serialize, Deserialize, Clone, Eq, PartialEq, Hash, Ord, PartialOrd, new)]
-pub enum Type {
-    Any,
+pub trait Type: std::fmt::Debug + Send {
+    fn name(&self) -> &'static str;
+    fn check(&self, value: Spanned<Value>) -> Result<Spanned<Value>, ShellError>;
+    fn coerce(&self) -> Option<hir::ExpressionKindHint> {
+        None
+    }
+}
+
+pub trait ExtractType<T>: Type {
+    fn extract(value: Value) -> T;
+}
+
+#[derive(Debug, Deserialize, Clone, Eq, PartialEq, Hash, Ord, PartialOrd, new)]
+pub struct Any;
+
+impl Type for Any {
+    fn name(&self) -> &'static str {
+        "Any"
+    }
+
+    fn check(&self, value: Spanned<Value>) -> Result<Spanned<Value>, ShellError> {
+        Ok(value)
+    }
+}
+
+#[derive(Debug, Deserialize, Clone, Eq, PartialEq, Hash, Ord, PartialOrd, new)]
+pub struct Integer;
+
+impl Type for Integer {
+    fn name(&self) -> &'static str {
+        "Integer"
+    }
+
+    fn check(&self, value: Spanned<Value>) -> Result<Spanned<Value>, ShellError> {
+        match value {
+            v @ Spanned {
+                item: Value::Primitive(Primitive::Int(_)),
+                ..
+            } => Ok(v),
+            other => Err(ShellError::type_error("Integer", other.spanned_type_name())),
+        }
+    }
+}
+
+impl ExtractType<i64> for Integer {
+    fn extract(value: Value) -> i64 {
+        match value {
+            Value::Primitive(Primitive::Int(int)) => int,
+            _ => unreachable!("invariant: must check before extract"),
+        }
+    }
+}
+
+#[derive(Debug, Deserialize, Clone, Eq, PartialEq, Hash, Ord, PartialOrd, new)]
+pub struct Block;
+
+impl Type for Block {
+    fn name(&self) -> &'static str {
+        "Block"
+    }
+
+    fn check(&self, value: Spanned<Value>) -> Result<Spanned<Value>, ShellError> {
+        match value {
+            v @ Spanned {
+                item: Value::Block(_),
+                ..
+            } => Ok(v),
+            other => Err(ShellError::type_error("Block", other.spanned_type_name())),
+        }
+    }
+}
+
+impl ExtractType<value::Block> for Block {
+    fn extract(value: Value) -> value::Block {
+        match value {
+            Value::Block(block) => block,
+            _ => unreachable!("invariant: must check before extract"),
+        }
+    }
 }
diff --git a/src/prelude.rs b/src/prelude.rs
index 7a9db241..9c79787c 100644
--- a/src/prelude.rs
+++ b/src/prelude.rs
@@ -40,7 +40,9 @@ crate use crate::context::Context;
 crate use crate::env::host::handle_unexpected;
 crate use crate::env::{Environment, Host};
 crate use crate::errors::ShellError;
+crate use crate::object::types::{ExtractType, Type};
 crate use crate::object::{Primitive, Value};
+crate use crate::parser::{Span, Spanned};
 crate use crate::stream::{InputStream, OutputStream};
 crate use crate::Text;
 crate use futures::stream::BoxStream;