Idris2-boot/TypeDD.md

Type Driven Development with Idris

The code in the book "Type Driven Development with Idris" by Edwin Brady, published by Manning, will mostly work in Idris 2, with some small changes as detailed in this document. The updated code is also [going to be] part of the test suite (see tests/typedd-book).

Chapter 1

No changes necessary

Chapter 2

The Prelude is smaller than Idris 1, and many functions have been moved to the base libraries instead. So:

In Average.idr, add:

import Data.Strings -- for `words`
import Data.List -- for `length` on lists

In AveMain.idr and Reverse.idr add:

import System.REPL -- for 'repl'

Chapter 3

Unbound implicits have multiplicity 0, so we can't match on them at run-time. Therefore, in Matrix.idr, we need to change the type of createEmpties and transposeMat so that the length of the inner vector is available to match on:

createEmpties : {n : _} -> Vect n (Vect 0 elem)
transposeMat : {n : _} -> Vect m (Vect n elem) -> Vect n (Vect m elem)

Chapter 4

For the reasons described above:

• In DataStore.idr, add import System.REPL and import Data.Strings

• In SumInputs.idr, add import System.REPL

• In TryIndex.idr, add an implicit argument:

  tryIndex : {n : _} -> Integer -> Vect n a -> Maybe a

Chapter 5

There is no longer a Cast instance from String to Nat, because its behaviour of returing Z if the String wasn't numeric was thought to be confusing. Instead, there is stringToNatOrZ in Data.Strings which at least has a clearer name. So:

In Loops.idr and ReadNum.idr add import Data.Strings and change cast to stringToNatOrZ

Chapter 6

In DataStore.idr and DataStoreHoles.idr, add import Data.Strings and import System.REPL. Also in DataStore.idr, the schema argument to display is required for matching, so change the type to:

display : {schema : _} -> SchemaType schema -> String

In TypeFuns.idr add import Data.Strings

Chapter 7

Abs is now a separate interface from Neg. So, change the type of eval to include Abs specifically:

eval : (Abs num, Neg num, Integral num) => Expr num -> num

Also, take abs out of the Neg implementation for Expr and add an implementation of Abs as follows:

Abs ty => Abs (Expr ty) where
    abs = Abs

Chapter 8

In AppendVec.idr, add import Data.Nat for the Nat proofs

cong now takes an explicit argument for the function to apply. So, in CheckEqMaybe.idr change the last case to:

checkEqNat (S k) (S j) = case checkEqNat k j of
                              Nothing => Nothing
                              Just prf => Just (cong S prf)

A similar change is necessary in CheckEqDec.idr.

In ExactLength.idr, the m argument to exactLength is needed at run time, so change its type to:

exactLength : {m : _} ->
              (len : Nat) -> (input : Vect m a) -> Maybe (Vect len a)

A similar change is necessary in ExactLengthDec.idr. Also, DecEq is no longer part of the prelude, so add import Decidable.Equality.

In ReverseVec.idr, add import Data.Nat for the Nat proofs.

Chapter 9

• In ElemType.idr, add import Decidable.Equality

In Hangman.idr:

• Add import Decidable.Equality and import Data.Strings

• guesses and letters are implicit arguments to game, but are used by the definition, so add them to its type:

  game : {guesses : _} -> {letters : _} -> WordState (S guesses) (S letters) -> IO Finished

Chapter 10

TODO

Chapter 11

TODO

Chapter 12

TODO

Chapter 13

TODO

Chapter 14

TODO

Chapter 15

TODO