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This project aims to create a next-generation, live programming environment that radically improves the programming experience.

See the Main Page


To build Lamdu from source, see the instructions for your platform


Note: In the shortcut key combinations, "left" refers to the left cursor key.

Simple expressions

At the top we have an interactive shell, where we can type calculations.

The is our prompt to this shell. Think of it like a calculator: you enter an expression, hit a button, and it tells you the answer. The next time you use the calculator, you clear whatever's in there and enter a new expression. Same here.

Golden ratio example

To type the calculation above:

  • Type "1+s" at the prompt (). Notice we have chosen "1" for the addition's left argument. However, we have only begun to type the second argument: it starts with an "s". Lamdu knows we have finalized the left argument because we have moved on from it, indicated by the +. But we have done nothing to indicate that just s is the second argument. To help us finalize the right argument, Lamdu has presented a menu of type-appropriate choices containing "s" in their names – "containing", not just "starting with". This menu updates as we type.
  • Next, we will flesh out the "s" into a "sqrt". As of September 2017, "sqrt" should already be selected in the nearby menu, because it is alphabetically the first function in the library to contain an "s" in its name and to output a number. However your menu, take the path of fewest keystrokes: continuing to type the function's name reduces the menu options to just those that match. Cursor keys allow you to select from the menu. Hit space to chose your selected menu option.
  • Type "5".
  • Select the whole expression by pressing shift+left until the whole REPL expression is selected.
  • Type "/2". Notice that Lamdu automatically inserted the parentheses.

Lamdu displays the evaluation of each expression, whether the whole or a subexpression. Such an automatic display is called an "annotation". The annotation of an expression appears below that of any child expression. For example, the evaluation of (1 + sqrt 5) / 2 appears below that of its child expression, (1 + sqrt 5). The former is 1.61... and the latter is 3.23....

To keep the expression size from bloating, some annotations are shrunk, like that of the sqrt 5 above, which is 2.23.... To see this in normal size, navigate to the expression by going to the sqrt, or to the 5, and press shift+left.

We have just expressed the golden ratio. To save it and give it a name, navigate to the sign and press return. Press return to name the new definition. Type "golden" and enter. You do not need to explicitly save - as your Lamdu program is always saved.

Creating a function

Note: Ctrl-Z is undo.

Factorial function

To create the function above:

  • Navigate to the "New..." button and press space.

Type factorial x=.

Note: Lamdu spaces your code automatically.

When you press space at the left-hand-side of a definition, Lamdu adds a parameter to the function and does not add a "space" as it would in a normal text editor.

The equals sign after factorial appears without typing it because all definitions have one. However, after factorial x, you may type an equals sign anyways, or skip over it with the right cursor key.

Now type the body of the function: if x=0 1 x*fac(x-1)

We've now written the function. Let's use it.

  • Go back up to the REPL, just right of the symbol. Like with calculators, we want to clear anything in there before using it. If there is an expression there, press shift+left until all is selected, then hit delete.
  • Type "fac 5" and press space.

Lamdu should now display the evaluation of the whole function, as well as its subexpresssions. The active if branch (the else) is highlighted via a green background on the | symbol. The | represents a suspended computation.

This function is recursive and invoked additional applications of itself. To navigate between these function applications, navigate to the arrows under the x parameter and press right or left.

Further Exploration / Help Documentation

In the lower-right of Lamdu's screen, you'll see that F1 brings up contextual help.

It shows all the key bindings currently active, which changes according to the current context.