### Define a function with a type error

Source: https://typedclojure.org/guides/getting_started

Example of code that will trigger a type error when checked.

```clojure
(ns org.my-domain.my-project
  (:require [typed.clojure :as t]))

(t/ann welcome-string [t/Str :-> t/Str])
(defn welcome-string [a-name]
  (str "Welcome, " a-name))

(welcome-string nil)
```

--------------------------------

### Create a new project

Source: https://typedclojure.org/guides/getting_started

Initialize a new project using Leiningen.

```bash
$ lein new org.my-domain/my-project
$ cd my-project
```

--------------------------------

### Configure project.clj

Source: https://typedclojure.org/guides/getting_started

Add Typed Clojure dependencies to your project configuration file.

```clojure
(defproject org.my-domain/my-project "0.1.0-SNAPSHOT"
  :dependencies [[org.clojure/clojure "1.11.1"]
                 ;; annotation macros like t/ann
                 [org.typedclojure/typed.clj.runtime "1.1.5"]]
  :profiles {:dev {:dependencies [;; the full type checker, only needed at dev-time
                                  [org.typedclojure/typed.clj.checker "1.1.5"]]}}
  :repl-options {:init-ns org.my-domain.my-project})
```

--------------------------------

### Run the type checker

Source: https://typedclojure.org/guides/getting_started

Execute the type checker within a running REPL session.

```bash
$ lein repl
Clojure 1.11.1
org.my-domain.my-project=> (t/check-ns-clj)
Start checking org.my-domain.my-project
Checked org.my-domain.my-project in 32.836713 msecs
:ok
```

--------------------------------

### Inspect type documentation

Source: https://typedclojure.org/guides/getting_started

Use the doc-clj function to retrieve information about specific types.

```clojure
org.my-domain.my-project=> (t/doc-clj t/U)
Special type: typed.clojure/U
U represents a union of types
Forms: [(U type*)]

nil
org.my-domain.my-project=> (t/doc-clj t/Str)
Type alias typed.clojure/Str
String
Metadata:
{:doc "A string", :forms '[Str], :file "typed/ann/clojure.cljc"}
nil
```

--------------------------------

### Annotate and define a function

Source: https://typedclojure.org/guides/getting_started

Define a function with explicit type annotations for arguments and return values.

```clojure
(ns org.my-domain.my-project
  (:require [typed.clojure :as t]))

(t/ann welcome-string [t/Str :-> t/Str])
(defn welcome-string [a-name]
  (str "Welcome to " a-name "!"))

(welcome-string "Typed Clojure")
;=> "Welcome to Typed Clojure!"
```

--------------------------------

### View type error output

Source: https://typedclojure.org/guides/getting_started

The output displayed in the REPL when a type mismatch is detected.

```text
org.my-domain.my-project=> (t/check-ns-clj)
Start checking org.my-domain.my-project
Type Error (file:/Users/ambrose/Projects/typedclojure.org/example-projects/my-project/src/org/my_domain/my_project.clj:10:1)
Function welcome-string could not be applied to arguments:


Domains:
        t/Str

Arguments:
        nil

Ranges:
        t/Str




in:
(welcome-string nil)



Execution error (ExceptionInfo) at clojure.core.typed.errors/print-errors! (errors.cljc:274).
Type Checker: Found 1 error
```

--------------------------------

### Verify successful type check

Source: https://typedclojure.org/guides/getting_started

Confirm the namespace passes type checking after updating annotations.

```clojure
org.my-domain.my-project=> (t/check-ns-clj)
Start checking org.my-domain.my-project
Checked org.my-domain.my-project in 32.831593 msecs
:ok
```

--------------------------------

### Update type annotation for nil

Source: https://typedclojure.org/guides/getting_started

Modify the annotation to allow nil values using a union type.

```clojure
(t/ann welcome-string [(t/U nil t/Str) :-> t/Str])
```

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