This term, I’m a TA for Oregon State University’s Programming Languages course.
The students in the course are tasked with using Haskell to implement a programming
language of their own design. One of the things they can do to gain points for the
project is implement type checking, rejecting
[note:
Whether or not the below example is ill-typed actually depends on your language.
Many languages (even those with a static type system, like C++ or Crystal)
have a notion of "truthy" and "falsy" values. These values can be used
in the condition of an if-expression, and will be equivalent to "true" or "false",
respectively. However, for simplicity, I will avoid including
truthy and falsy values into the languages in this post. For the same reason, I will avoid
reasoning about
type coercions,
which make expressions like `"Hello"+3`

valid.
]
such as:

```
if "Hello" then 0 else 1
```

For instance, a student may have a function `typecheck`

, with the following
signature (in Haskell):

```
typecheck :: Expr -> Either TypeError ExprType
```

The function will return an error if something goes wrong, or, if everything goes well, the type of the given expression. So far, so good.

A student asked, however:

Now that I ran type checking on my program, surely I don’t need to include errors in my [note: I'm using "valuation function" here in the context of denotational semantics. In short, a valuation function takes an expression and assigns to it some representation of its meaning. For a language of arithmetic expression, the "meaning" of an expression is just a number (the result of simplifying the expression). For a language of booleans,

`and`

, and`or`

, the "meaning" is a boolean for the same reason. Since an expression in the language can be ill-formed (like`list(5)`

in Python), the "meaning" (semantic domain) of a complicated language tends to include the possibility of errors. ] I should be able to make my function be of type`Expr -> Val`

, and not`Expr -> Maybe Val`

!

Unfortunately, this is not quite true. It is true that if the student’s type checking
function is correct, then there will be no way for a type error to occur during
the evaluation of an expression “validated” by said function. The issue is, though,
that **the type system does not know about the expression’s type-correctness**. Haskell
doesn’t know that an expression has been type checked; worse, since the function’s type
indicates that it accepts `Expr`

, it must handle invalid expressions to avoid being partial. In short, even if we **know** that the
expressions we give to a function are type safe, we have no way of enforcing this.

A potential solution offered in class was to separate the expressions into several
data types, `BoolExpr`

, `ArithExpr`

, and finally, a more general `Expr'`

that can
be constructed from the first two. Operations such as `and`

and `or`

will then only be applicable to boolean expressions:

```
data BoolExpr = BoolLit Bool | And BoolExpr BoolExpr | Or BoolExpr BoolExpr
```

It will be a type error to represent an expression such as `true or 5`

. Then,
`Expr'`

may have a constructor such as `IfElse`

that only accepts a boolean
expression as the first argument:

```
data Expr' = IfElse BoolExpr Expr' Expr' | ...
```

All seems well. Now, it’s impossible to have a non-boolean condition, and thus,
this error has been eliminated from the evaluator. Maybe we can even have
our type checking function translate an unsafe, potentially incorrect `Expr`

into
a more safe `Expr'`

:

```
typecheck :: Expr -> Either TypeError (Expr', ExprType)
```

However, we typically also want the branches of an if-expression to both have the same
type - `if x then 3 else False`

may work sometimes, but not always, depending of the
value of `x`

. How do we encode this? Do we have two constructors, `IfElseBool`

and
`IfElseInt`

, with one in `BoolExpr`

and the other in `ArithExpr`

? What if we add strings?
We’ll be copying functionality back and forth, and our code will suffer. Wouldn’t it be
nice if we could somehow tag our expressions with the type they produce? Instead of
`BoolExpr`

and `ArithExpr`

, we would be able to have `Expr BoolType`

and `Expr IntType`

,
which would share the `IfElse`

constructor…

It’s not easy to do this in canonical Haskell, but it can be done in Idris!

### Enter Dependent Types

Idris is a language with support for dependent types. Wikipedia gives the following definition for “dependent type”:

In computer science and logic, a dependent type is a type whose definition depends on a value.

This is exactly what we want. In Idris, we can define the possible set of types in our language:

```
data ExprType
= IntType
| BoolType
| StringType
```

Then, we can define a `SafeExpr`

type family, which is indexed by `ExprType`

.
Here’s the
[note:
I should probably note that the definition of `SafeExpr`

is that of
a
Generalized Algebraic Data Type,
or GADT for short. This is what allows each of our constructors to produce
values of a different type: `IntLiteral`

builds `SafeExpr IntType`

,
while `BoolLiteral`

builds `SafeExpr BoolType`

.
]
which we will discuss below:

```
data SafeExpr : ExprType -> Type where
IntLiteral : Int -> SafeExpr IntType
BoolLiteral : Bool -> SafeExpr BoolType
StringLiteral : String -> SafeExpr StringType
BinOperation : (repr a -> repr b -> repr c) -> SafeExpr a -> SafeExpr b -> SafeExpr c
```

The first line of the above snippet says, “`SafeExpr`

is a type constructor
that requires a value of type `ExprType`

”. For example, we can have
`SafeExpr IntType`

, or `SafeExpr BoolType`

. Next, we have to define constructors
for `SafeExpr`

. One such constructor is `IntLiteral`

, which takes a value of
type `Int`

(which represents the value of the integer literal), and builds
a value of `SafeExpr IntType`

, that is, an expression that **we know evaluates
to an integer**.

The same is the case for `BoolLiteral`

and `StringLiteral`

, only they build
values of type `SafeExpr BoolType`

and `SafeExpr StringType`

, respectively.

The more complicated case is that of `BinOperation`

. Put simply, it takes
a binary function of type `a->b->c`

(kind of), two `SafeExpr`

s producing `a`

and `b`

,
and combines the values of those expressions using the function to generate
a value of type `c`

. Since the whole expression returns `c`

, `BinOperation`

builds a value of type `SafeExpr c`

.

That’s almost it. Except, what’s up with `repr`

? We need it because `SafeExpr`

is parameterized by a **value** of type `ExprType`

. Thus, `a`

, `b`

, and `c`

are
all values in the definition of `BinOperation`

. However, in a function
`input->output`

, both `input`

and `output`

have to be **types**, not values.
Thus, we define a function `repr`

which converts values such as `IntType`

into
the actual type that `eval`

would yield when running our expression:

```
repr : ExprType -> Type
repr IntType = Int
repr BoolType = Bool
repr StringType = String
```

The power of dependent types allows us to run `repr`

inside the type
of `BinOp`

to compute the type of the function it must accept.

Now, we have a way to represent expressions that are guaranteed to be type safe.
With this, we can make our `typeheck`

function convert an `Expr`

to a `SafeExpr`

.
Wait a minute, though! We can’t *just* return `SafeExpr`

: it’s a type constructor!
We need to somehow return `SafeExpr a`

, where `a`

is a value of type `ExprType`

. But
it doesn’t make sense for the return type to have a new type variable that didn’t
occur in the rest of the type signature. It would be ideal if we could return both
the type of the expression, and a `SafeExpr`

of that type.

In fact, we can!

Idris has something called *dependent pairs*, which are like normal pairs, but
in which the type of the second element depends on the value of the first element.
The canonical example of this is a pair of (list length, list of that many elements).
For instance, in Idris, we can write:

```
listPair : (n : Nat ** Vec n Int)
```

In the above snippet, we declare the type for a pair of a natural number (`n : Nat`

) and a list of
integers (`Vect n Int`

), where the number of elements in the list is equal to the natural number. Let’s
try applying this to our problem. We want to return an `ExprType`

, and a `SafeExpr`

which
depends on that `ExprType`

. How about this:

`typecheck : Expr -> Either String (n : ExprType ** SafeExpr n)`

Given an expression, we return either an error (`String`

) or a dependent pair, which
contains some `ExprType`

`n`

and a `SafeExpr`

that evaluates to a value of type `n`

.
We can even start implementing this function, starting with literals:

```
typecheck (IntLit i) = Right (_ ** IntLiteral i)
typecheck (BoolLit b) = Right (_ ** BoolLiteral b)
typecheck (StringLit s) = Right (_ ** StringLiteral s)
```

Note the use of `_`

. Since `IntLiteral`

always produces `SafeExpr IntType`

, we allow
Idris to infer that `IntType`

must be the first element of the tuple. This is easy
enough, because a boolean, integer, or string literal can never be type-incorrect.

The interesting case is that of a binary operation. Is `"hello" * 3`

invalid? It might
be, but some languages evaluate the multiplication of a string by a number as repeating
the string that many times: `"hellohellohello"`

. It is up to us, the language designers,
to specify the set of valid operations. Furthermore, observe that `BinOperation`

takes a *function* as its first argument, not an `Op`

. To guarantee that we can,
in fact, evaluate a `BinOperation`

to the promised type, we require that the means
of performing the evaluation is included in the expression. Thus, when we convert `Expr`

to `SafeExpr`

, we need to convert an `Op`

to a corresponding function. As we can see
with `"hello"*3`

and `163*2`

, an `Op`

can correspond to a different function
depending on the types of its inputs. To deal with this, we define a new function `typecheckOp`

,
which takes an `Op`

and two expression types, and returns either an error (if the `Op`

can’t
be applied to those types) or a dependent pair containing the output type of the operation
and a function that performs the required computation. That’s a mouthful; let’s look at the code:

```
typecheckOp : Op -> (a : ExprType) -> (b : ExprType) -> Either String (c : ExprType ** repr a -> repr b -> repr c)
typecheckOp Add IntType IntType = Right (IntType ** (+))
typecheckOp Subtract IntType IntType = Right (IntType ** (-))
typecheckOp Multiply IntType IntType = Right (IntType ** (*))
typecheckOp Divide IntType IntType = Right (IntType ** div)
typecheckOp _ _ _ = Left "Invalid binary operator application"
```

When `(+)`

is applied to two integers, this is not an error, and the result is also
an integer. To perform addition, we use Idris’ built-in function `(+)`

. The
same is true for all other arithmetic operations in this example. In all other
cases, we simply return an error. We can now use `typecheckOp`

in our `typecheck`

function:

```
typecheck (BinOp o l r) = do
(lt ** le) typecheck l
(rt ** re) typecheck r
(ot ** f) typecheckOp o lt rt
pure (_ ** BinOperation f le re)
```

Here, we use do-notation to first type check first the left, then the right subexpression.
Since the result of type checking the subexpressions gives us their output types,
we can feed these types, together with `o`

, to `typecheckOp`

to determine the output
type and the applicable evaluation function. Finally, we assemble the new `SafeExpr`

from the function and the two translated subexpressions.

Alright, we’ve done all this work. Is it worth it? Let’s try implementing `eval`

:

```
eval : SafeExpr t -> repr t
eval (IntLiteral i) = i
eval (BoolLiteral b) = b
eval (StringLiteral s) = s
eval (BinOperation f l r) = f (eval l) (eval r)
```

That’s it! No `Maybe`

, no error cases. `eval`

is completely total, but doesn’t require
error handling because it **knows that the expression it is evaluating is type-correct**!

Let’s run all of this. We’ll need some code to print the result of evaluating an expression. Here’s all that:

```
resultStr : {t : ExprType} -> repr t -> String
resultStr {t=IntType} i = show i
resultStr {t=BoolType} b = show b
resultStr {t=StringType} s = show s
tryEval : Expr -> String
tryEval ex =
case typecheck ex of
Left err => "Type error: " ++ err
Right (t ** e) => resultStr $ eval {t} e
main : IO ()
main = putStrLn $ tryEval $ BinOp Add (IntLit 6) (BinOp Multiply (IntLit 160) (IntLit 2))
```

And the output is:

```
>>> idris TypesafeIntr.idr -o typesafe
>>> ./typesafe
326
```

That’s right! What about a type-incorrect example?

```
BinOp Add (IntLit 6) (BinOp Multiply (IntLit 160) (StringLit "hi"))
```

The program reports:

```
Type error: Invalid binary operator application
```

Awesome!

### Wrapping Up

In this post, we learned that type checking can be used to translate an expression into a more strongly-typed data type, which can be (more) safe to evaluate. To help strengthen the types of [note: You may be thinking, "but where did the if-expressions go?". It turns out that making sure that the branches of an if-expression are of the same type is actually a fairly difficult task; the best way I found was enumerating all the possible "valid" combinations of types in a case-expression. Since this is obviously not the right solution, I decided to publish what I have, and look for an alternative. If I find a better solution, I will write a follow-up post. ] we used the Idris language and its support for dependent types and Generalized Algebraic Data Types (GADTs). I hope this was interesting!

As usual, you can find the code for this post in this website’s Git repository. The source file we went through today is found here.