2.2.3.2 J Programs

A single J sentence is also a program. For example:

   (4 + 5) * (3 - 2)
9

The J word "=:" assigns (binds) a name (pronoun) to a value (noun). For example, suppose we have defined the following words:

   monad =: 3
   define =: :

Then the sentence:

   square =: monad define 'y. * y.'

is an explicit definition for the square function. The pronoun "y." always refers to the input of a monad. Given this definition, then we may write:

   square 10
100

The J notation provides great expressive power when defining functions through an alternate method called tacit definition. A feature of tacit definition is there is no reference to the inputs of a definition. For example, square could also be defined as:

   square =: ^&2

Here we are using a conjunction (a verb producing verb) called bond (spelled ``SPMamp;'') which takes a verb input on the left (power function, spelled ``^'') and a noun input (2) on the right and produces a new verb which squares its argument. Square could also be defined as:

   square =: *~

Here we use the adverb reflex (spelled ``~'') which takes a dyad as its left input and converts the dyad into a monad by using its input y. as its left and right input for the dyad. In this case the verb derived from *  y. is y. * y. .

Finally, the square function occurs frequently enough in programs so that it is provided as a primitive function in J, (spelled "*:". So we could also write:

   square =: *: