hi, the Hobbes interpreter

In the bin directory of your Hobbes build, you’ll find the Hobbes interactive interpreter, “hi”. You can use it to execute much of the code you see on these pages by just typing the line and hitting ‘enter’. It’s an example of a REPL, or “Read, Eval, Print Loop” - because those are the key steps that allow it to execute the code you enter.

When used interactively, Hi will show you the results of the execution of a line immediately:

hobbes/bin $ hi

hi : an interactive shell for hobbes
  type ':h' for help on commands

> true
true
> 1 + 2.3
3.3

Throughout this documentation we’ll show the Hi prompt (‘>’) to indicate text you can type into Hi. You don’t need to type the prompt too.

hi can read files

If you pass the name of a text file to the Hobbes interpreter hi, it’ll evaluate the code in the file and make symbol names available to you in the hi session:

bin/hobbes $ cat match.hob
foo = match 3 with
  | 1 -> "hello"
  | 2 -> "hobbes"
  | _ -> "oops"

bin/hobbes $ hi match.hob
hi : an interactive shell for hobbes
  type ':h' for help on commands

loaded module 'match.hob'
> foo
"oops"

This makes it much easier to write more complex, multi-line expressions of the kind you’ll see in later sections. Some of these examples are therefore shown without the hi prompt.

Type information

Hobbes is a strong, statically-typed language, meaning every expression has a type which is known at compile time. The Hobbes compiler makes this type information available to the REPL Hi.

:t shows us the Hobbes type of the defined function:

> add = (\a b.a+b)
> :t add
Add a b c => (a * b) -> c

In this case, Hi shows us that we’ve implicitly defined add as an implementation of the typeclass Add.

Digging a little further, we can use :c to see the implementation of the Add typeclass, which is defined as any type which has an + member:

> :c Add
class Add | #0 #1 -> #2 where
  + :: (#0 * #1) -> #2

We can see all implemetations of a typeclass with :i:

> :i Add

instance Add char char char where
  + = cadd::(char * char) -> char::(char * char) -> char
instance Add byte byte byte where
  + = badd::(byte * byte) -> byte::(byte * byte) -> byte
instance Add short short short where
  + = sadd::(short * short) -> short::(short * short) -> short
...

Timing information

Some timing data can be retrieved for the complation and evaluation phases of a Hobbes expression:

> :e add(1, 3)
average over 1000 runs: 43ns
minimum runtime: 42ns
maximum runtime: 131ns

Commandline options

-s	Silent mode - don’t display the prelude or version information
-p nnnn	Listen on port nnnn