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+# REPL
+
+The REPL is a very helpful feature for functional programming languages
+like bruijn. You can use it to continuously test or execute parts of
+your code.
+
+You can start the REPL using `stack run`{.bash} or (if installed)
+`bruijn`{.bash}.
+
+Any valid term will get reduced to normal form after pressing enter.
+
+## Definitions
+
+Since everything you type will get evaluated, definitions (compared to
+definitions in files) require an equal sign:
+
+``` bruijn
+> id = [0]
+> id
+[0]
+```
+
+## Commands
+
+### `:import`{.bruijn}/`:input`{.bruijn}
+
+Equivalent to the [respective commands in
+files](../introduction/syntax.md#imports).
+
+``` bruijn
+> :import std/Math .
+```
+
+### `:test`{.bruijn}
+
+Equivalent to the [test command in
+files](../introduction/syntax.md#tests).
+
+``` bruijn
+> :test ([0]) ([[1]])
+ERROR test failed: [0] = [[1]]
+      reduced to [0] = [[1]]
+```
+
+### `:watch`{.bruijn}
+
+`:watch`{.bruijn} re-imports the file automatically after every saved
+change. It will rerun any test the watched file contains.
+
+``` bruijn
+> :watch collatz-proof
+```
+
+This can be very helpful for [test driven
+development](test-driven-development.md).
+
+### `:time`{.bruijn}
+
+Measures the time from start of reduction to its end (normal form) in
+seconds.
+
+``` bruijn
+> :time fac (+30)
+0.15 seconds
+```
+
+### `:blc`{.bruijn}
+
+Translates both the unreduced and the reduced expression to binary
+lambda calculus. Helpful for golfed [compilation](compilation.md).
+
+``` bruijn
+> :blc [0] [0]
+0100100010
+0010
+```
+
+### `:length`{.bruijn}
+
+Measures the length of the binary lambda calculus encoding of both the
+unreduced and the reduced expression. Helpful for golfed
+[compilation](compilation.md).
+
+``` bruijn
+> :blc [0] [0]
+0100100010
+0010
+```
+
+### `:free`{.bruijn}
+
+The `free` command frees the current environment including all defined
+identifiers and imported files.
+
+``` bruijn
+> id = [0]
+> :free
+> id
+ERROR undefined identifier id
+```