Control Flow

The Hobbes language itself is quite small. Function definitions, as well as types, type classes, and their instances, must be written in a Hobbes file in order to be defined. These examples are therefore shown without the hi prompt - for more examples, see hi can load files

The control flow rules for Hobbes are similar to Haskell:

Functions

Functions are first declared by their type, and then implemented for specific values.

addOne :: int -> int
addOne s = s + 1

If we didn’t first specify the type of the function, Hobbes would attempt to create a polymorphic function with parameter types restrcited simply by the way in which values of that type are used. For more about this behaviour, see polymorphism.

If/else

if a < 1 then 2 else 3

Note

Note that ‘if’ is an expression, meaning that it resolves to a value:

> b = if true then 12 else 13
> b
12

Pattern matching

Match expressions are perhaps the most powerful element of Hobbes syntax, because they let you perform actions based on the value or even type of other expressions.

In the simplest form, they’re a bit like a switch statement:

match 3 with
| 1 -> show("hello")
| 2 -> show("hobbes")
| _ -> show("oops!")

Note

underscores In Hobbes match expressions, the underscore serves two purposes.

Firstly, it’s a wildcard. In the above code the underscore is the default case that will be executed if all the above cases have failed to match.

Secondly, it’s an instruction not to bind a name to the matched element. We’ll see more about binding below!

When used with more than one value they can be used to match against any element in the set. However, the Hobbes compiler is smart, and will complain if you haven’t provided a case for all the potential options. In the next example, we’ll be caught red-handed with a so-called “inexhaustive match”:

match 1 2 with
| 2 _ -> show("first!")
| 1 2 -> show("second!")

Warning

Inexhaustive patterns in match expression

We can always resolve this by adding a default case, or by providing cases for all the possible options - although this might mean writing a lot of code!

match 1 2 with
| 2 _ -> show("first!")
| 1 2 -> show("second!")
| _ _ -> show("default!")

In the above example, it’s important to note that the matching works top-down, meaning that the first valid case will be evaluated:

match 1 2 with
| _ 2 -> show("matched!")
| 1 2 -> show("didn't match!")
| _ _ -> show("didn't even get here!")

The way to read the first case is “any value followed by the integer 2”. Even though the second match is more specific (i.e., both elements match the values), it’s the first case that’s matched.

Also, note that because we’re matching against two values, we have to use two underscores in the final case. If we fail to do that, Hobbes will tell us “row #3 has 1 columns, but should have 2”.

Note

Just as if expressions can be written on one line, we can save space (and be more idiomatic) in our Hobbes code in the same way. The above match can be re-written as

match 1 2 with | _ 2 -> show("matched!") | 1 2 -> show("didn't match!") | _ _ -> show("didn't even get here!")

The Hobbes standard library is full of code like this, and Hobbes developers quickly get used to writing code this terse. You can decide what works best for you!

Matching and binding

As well as matching on values, we can also bind values to names within a match case. In the following example, we’re matching on the first element of the tuple and binding to the second:

match 'a' 123 with
| 'a' fst -> show(fst)
| 'b' snd -> show(snd)
| _ _ -> show("default")

In each case, we’re simply matching on the (char) value of the first element. If that matches, we bind the second element to a value. In the first case (which ultimately is matched), the name we give the value is fst, but there’s nothing special about that; we could have called it anything. The name fst is then lexically scoped to the match expression following the arrow - it’s not available in other cases, or outside the match.

Note

To some programmers, this “match and bind” behaviour seems strange, and it’s another good example of the “terse vs powerful” battle often found in the minds of new functional programmers!

Tuples

Hobbes also lets us match against the values of tuple elements, leading to another common idiom. The ease with which we can match and bind using the match syntax with tuples means that ad-hoc tuples are often created simply to limit pollution of the global namespace with values which could be scoped more appropriately. Consider the below case:

match env getHostPort(env) with
| "dev" (host, port) -> connect(host, port)
| "qa" (host, port) -> connectqa(host, port, qadb)
| "prod" (host, _) -> connectkrb(host)
| _ _ -> ...

In this case we’re creating a tuple simply for the purposes of immediately matching against its values and unpacking it.

Here again the underscore is used as a wildcard - in this case you can read it to mean “there is a value here but I don’t care what it is, and I don’t want to use it so don’t even give it a name”.

This matching-and-binding logic can be generalised to arrays, too:

match [("sam", 2013), ("james", 2012), ("stephen", 2010)] with
| [_ (n, 2012), _] -> show(n)
| _ -> show("none")

And, because of the way character arrays are matched, even to regular expressions:

match hostname with
| '.*qa$' -> show("qa")
| _ -> show("prod")

Guard matching

We can also match based on ranges of values, using a so-called “guard”:

match 1 with
| x where x < 10 -> show("small!")
| _ -> show("large!")

Note

The rules for match expressions are simple: every case in the expression must be reachable (i.e., no previous row can have matched against all the possible values for this row) and the match table must be exhaustive (i.e. all possible cases must be matched against).

These rules combined explain why you so commonly see wildcard matches at the end of a match expression - the wildcard catches any cases that haven’t previously been matched; and putting it at the end it prevents further cases from being unreachable.

Remember, the rule is first possible match, not most specific match!

Matching on Variants

Just like with tuples we can match on - and unpack - sum and variant types. Recall our status type from earlier:

type status = | Succeess, Failure: int|

We can write a matching function which classifies values of this type and acts accordingly:

classify :: status -> [char]
classify s = match s with
| |Success| -> "finished"
| |Failure=x| -> "failed with error" ++ show(x)

Similarly to the complex match expressions above, we can match on values as well, to provide special functionality for specific cases:

classify :: status -> [char]
classify s = match s with
| |Success| -> "Succeeded"
| |Failure=404| -> "Not Found"
| |Failure=err| -> "Error: " ++ show(err)

Match expressions

In the previous examples, we’ve been calling the unit show function in our match cases. But in Hobbes, just like with if, match is an expression - that means it’s results can be assigned directly to a name:

hostport = match env with | "prod" -> "lnprd" | "qa" -> "euqa" | _ -> "ln123dev"

And just like with if, the types of the expressions in each branch must also match. Failing to ensure the cases are of the same type will result in an error:

> match 1 with | 0 -> "hello" | _ -> 1
Cannot unify types: [char] != int

Sugar

The Hobbes compiler uses match expressions “under the covers” in a number of different situations. For example, the matches keyword can be used to perform all the unpacking and pattern-matching that a single-case match statement can:

(1, 2) matches (1, 2)

is re-written by the Hobbes compiler to

match (1, 2) with
| (1, 2) -> true
| _ -> false

Similarly, these two are equivalent in Hobbes:

"sam" matches '..m'

match "sam" with
| '..m' -> true
| _ -> false

This process of conversion to another program structure is commonly called “desugaring”, because the nicer, lighter-weight style is known as “syntactic sugar”. There are many examples of sugaring in the Hobbes language, and we’ll try to point them out as we go. Sweet!

Tuple Decomposition

A tuple can be decomposed into its individual parts very simply:

> (host, port) = getHostPort("dev")
> host
"lndev01"
> :t port
int

Comprehensions

Similar to comprehensions in Python, these allow us to describe the algorithm used to create a sequence of data.

Note

Remember, Hobbes code is executed eagerly, meaning the comprehension will usually be evaluated in full when it is declared.

[show(x) | x <- [0..20], x % 3 == 0]

This can be read as “for each x in 0 to 20, where x is divisible by 3, show x”.

The comprehension is split into a mapping function, a generator expression, and a filter. The mapping function is applied to the results of the generator function where the filter holds true.

Note

sequence expressions

Look closely and you’ll see the sequence expression syntax described in the types chapter. You could also use an inline array declaration here:

> [show(x) | x <- [11, 12, 13], x % 3 == 0]

Or alternatively the name of an array declared elsewhere:

> nums = [98, 99, 100]
> [show(x) | x <- nums, x % 3 == 0]

The comprehension syntax is an expression, and can therefore be used anywhere a range of elements is expected. For an example, the Hobbes standard library contains the following code:

productWith :: ((a, b) -> c, [a], [b]) -> [c]
productWith f xs ys = concat([[f(x,y) | y <- ys] | x <- xs])

This describes a function productWith, which combines the cross product of elements from two lists with a function:

> productWith((\x y.x+y),[1,2,3],[4,5,6])
[5, 6, 7, 6, 7, 8, 7, 8, 9]

If we were to write out this productWith function in pseudocode, it might look like this:

for(x in [1, 2, 3])
  for(y in [4, 5, 6])
    yield (x, y)

Local variables

In order to avoid polluting the Hobbes global namespace, we can declare variables as local to the current expression:

> let x = 9 in x * x
81
> x
stdin:1,1-1: Undefined variable: 'x' ...

In this case the name x is only in scope in the following expression. This allows us to re-use names without having to deal with x1, x2, etc.

Let expressions can allow more than one local variable to be declared:

> let x=1; y=10 in x + y
11

Indeed, let expressions are very powerful. In the following example we’re first declaring, and then decomposing a tuple before the expression itself is evaluated:

> let f = (\x.(x,x)); (x, y) = f(20) in x + y
40

Notice how we’re even able to reuse the name x across both the function declaration and the resultant tuple deconstruction. Let expressions are evaluated in declaration order, before the execution primary expression.

Let allows us to unpack tuple values in a convenient format:

> hostport = ("lndev1", 234)
> let (h, p) = hostport in show(p)
234

Note

This form of let is actually converted into a simple match for us by the Hobbes compiler:

match hostport with
| (h, p) -> show(p)

Note that this “de-sugaring” will only take place if the compiler can determine that the match can never fail.

Finally, let’s wrap all that up with a match and a for comprehension:

> let start=1; end=4 in match [i | i <- [start..end], i % 2 == 0] with | [2, 4] -> "evens" | _ -> "odds"
evens