Views and the “with” rule

Dependent pattern matching

Since types can depend on values, the form of some arguments can be determined by the value of others. For example, if we were to write down the implicit length arguments to (++), we’d see that the form of the length argument was determined by whether the vector was empty or not:

(++) : Vect n a -> Vect m a -> Vect (n + m) a
(++) {n=Z}   []        ys = ys
(++) {n=S k} (x :: xs) ys = x :: xs ++ ys

If n was a successor in the [] case, or zero in the :: case, the definition would not be well typed.

The with rule — matching intermediate values

Very often, we need to match on the result of an intermediate computation. Idris provides a construct for this, the with rule, inspired by views in Epigram [1], which takes account of the fact that matching on a value in a dependently typed language can affect what we know about the forms of other values. In its simplest form, the with rule adds another argument to the function being defined, e.g. we have already seen a vector filter function, defined as follows:

filter : (a -> Bool) -> Vect n a -> (p ** Vect p a)
filter p [] = ( _ ** [] )
filter p (x :: xs) with (filter p xs)
  | ( _ ** xs' ) = if (p x) then ( _ ** x :: xs' ) else ( _ ** xs' )

Here, the with clause allows us to deconstruct the result of filter p xs. Effectively, it adds this value as an extra argument, which we place after the vertical bar.

If the intermediate computation itself has a dependent type, then the result can affect the forms of other arguments — we can learn the form of one value by testing another. For example, a Nat is either even or odd. If it’s even it will be the sum of two equal Nat. Otherwise, it is the sum of two equal Nat plus one:

data Parity : Nat -> Type where
   Even : Parity (n + n)
   Odd  : Parity (S (n + n))

We say Parity is a view of Nat. It has a covering function which tests whether it is even or odd and constructs the predicate accordingly.

parity : (n:Nat) -> Parity n

We’ll come back to the definition of parity shortly. We can use it to write a function which converts a natural number to a list of binary digits (least significant first) as follows, using the with rule:

natToBin : Nat -> List Bool
natToBin Z = Nil
natToBin k with (parity k)
   natToBin (j + j)     | Even = False :: natToBin j
   natToBin (S (j + j)) | Odd  = True  :: natToBin j

The value of the result of parity k affects the form of k, because the result of parity k depends on k. So, as well as the patterns for the result of the intermediate computation (Even and odd) right of the |, we also write how the results affect the other patterns left of the |. Note that there is a function in the patterns (+) and repeated occurrences of j—this is allowed because another argument has determined the form of these patterns.

We will return to this function in Section Provisional Definitions to complete the definition of parity.

[1]Conor McBride and James McKinna. 2004. The view from the left. J. Funct. Program. 14, 1 (January 2004), 69-111. DOI=10.1017/S0956796803004829ñ