edwinb / typedd-samples

Sample code from "Type Driven Development with Idris"

Type Driven Development with Idris

Sample code and exercise solutions from "Type Driven Development with Idris", available from https://www.manning.com/books/type-driven-development-with-idris

typedd-samples's Issues

Listing 6.8 `toFormat` '%' case

Hi Edwin,

first of all, thank you for writing such a brilliant book, it's a pleasure to read!

Secondly, I have a question regarding the implementation of toFormat (listing 6.8). It currently goes like this (Chapter6/Printf.idr):

toFormat : (xs : List Char) -> Format
toFormat [] = End
toFormat ('%' :: 'd' :: chars) = Number (toFormat chars)
toFormat ('%' :: 's' :: chars) = Str (toFormat chars)
toFormat ('%' :: chars) = Lit "%" (toFormat chars)
toFormat (c :: chars) = case toFormat chars of
                             Lit lit chars' => Lit (strCons c lit) chars'
                             fmt => Lit (strCons c "") fmt

I don't understand the reason behind the toFormat ('%' :: chars) case. Shouldn't we simply get rid of it and let '%' fall into the final case where it will, if needed, nicely be appended inside a Lit?

For example, given the input "foo%bar", with the implementation in the book, you get:

> toFormat . unpack $ "foo%bar"
Lit "foo%" (Lit "bar" End) : Format

Whereas, when removing the toFormat ('%' :: chars) case, you get:

> toFormat . unpack $ "foo%bar"
Lit "foo%bar" End : Format

I think the latter makes more sense but maybe I am missing something. Happy to open a PR if needed.

Cheers!
Bastien

^D in the average word length example causes weird behavior

In this example, https://github.com/edwinb/TypeDD-Samples/blob/master/Chapter2/AveMain.idr, entering ^D just cause an infinite stream of "Enter a string: The average word length is: nan". Is this expected or a bug?

I expect it to show "Enter a string: The average word length is: nan" just once.

Can't find implementation of Ord a

I am new to Idris, so forgive me if this a dumb question.

When doing Exercise 5 on page 101 of the TypeDD book, my solution (which is the same as the suggested solution in this repository) exhibits the following behavior when I launch idris ex_4_1.idr:

Type checking ./ex_4_1.idr
*ex_4_1> maxMaybe Nothing Nothing
Can't find implementation for Ord a

According to the definition of maxMaybe (included below for reference), it seems the answer should be Nothing. Is this an error? Do I not have Idris set up correctly?

maxMaybe : Ord a => Maybe a -> Maybe a -> Maybe a
maxMaybe Nothing Nothing = Nothing
maxMaybe Nothing (Just y) = Just y
maxMaybe (Just x) Nothing = Just x
maxMaybe (Just x) (Just y) = Just (max x y)

Wrong word in Ch13/Vending.idr

in line 91,
do Just x <- GetInput | Nothig => do Display "Invalid input"
Nothig -> Nothing

definition of ProcState in Chapter 15

It's mildly confusing, but in the book, starting from section 15.2.3, the following definition is given:

data ProcState = NoRequest | Sent | Complete

whereas in this repo, ProcessIFace.idr, ProcessLib.idr, ProcessState.idr define it as

data ProcState = Ready | Sent | Looping

How does the inferring work on this line?

TypeDD-Samples/Chapter6/Printf.idr

Line 27 in a5c08a1

printf fmt = printfFmt _ ""

Use an underscore (_) for the format, because Idris can infer from the type that it must be toFormat (unpack fmt).

This's the explanation from the book for above line but I'm still a bit puzzled by how does the inferring work here? Since fmt comes in as a String type, how does it become a Format type when passed to printfFmt?

listing 11.31

In listing 11.31, this:

runCommand (Bind c f) = do res <- runCommand c
                           runCommand (f res)

should be

runCommand (Bind c f) = do res <- runCommand (Pure c)
                           runCommand (f res)