7 ms·

> [...] Idris is a lot more practical than I thought. This may sound like a stupid question (but is probably necessary in a thread about the convergence of pro

> [...] Idris is a lot more practical than I thought. This may sound like a stupid question (but is probably necessary in a thread about the convergence of proof systems and static typed functional programming languages): "How can i implement something like unix' wc?" So just a program that counts lines (forget words and bytes for now) of a file. This means we need to support (at least in some kind of standard library) basic IO stuff like files, processes and environments. If the language talks about how "simple" (in terms of code/term length) it is to define a binary tree ... the integers >= 0 (the famous Nat = Zero | Succ Nat) example, or beware the Fibonacci sequence - most people won't be impressed. Because that's what is associated with "programming language" (as compared to "proof assistents"). Sorry that sounds a bit like a rant, but after 5 minutes of searching I couldn't find an example how to read a file in Idris. The closest I found was: http://docs.idris-lang.org/en/latest/effects/impleff.html http://docs.idris-lang.org/en/latest/effects/impleff.html but it did not tell me how to open a file ...

seanmcdirmid 7y ago
I don’t know about Idris, but the basic idea would be transform the file’s type from (a|b)* into (a* b)* a* where b is a newline and a is any non newline. Then your answer simply counts the first star of the transformed type (change that star to a counter exponent as you are forming the type).
- zeptomu 7y ago
  Yeah ... but how does that look? A "naive" wc implementation (counting the character '\n') is <50 lines of code (if we have modern file IO in the standard library) in a practical PL. But maybe there is a too big difference between the concept of "files" (as seen by OS developers that view them as a byte-blob in a hierarchical name system) and language designers that want to represent a file as a collection of lines (and probably would like your command line interpreter not allow to call "wc" on a file that contains non UTF8 encoded data) - but this is still a pretty big step for our current computation environment (where I can call programs by name that are also files). For many people (including me) a "practical" programming language needs an implementation that can generate (compile) an executable or interpret a file representation of its source. Otherwise its a proof assistant. Maybe we have to merge the concept of data in terms (no pun indented) of bytes (types?) in-memory and collections of bits and bytes that have a lifetime longer than the program (if it's not a daemon)?
  - seanmcdirmid 7y ago
    If you implement by manipulating the types directly, a few operations with a loop (just for counting lines mind you). Again, I’m not sure how this would be done in Agda, Idris, or COQ, and the system designing is a bit different from other type systems. But also, any discrete one dimensional problem like wc is going to benefit from a type system with types that resembles regular expressions.
  - chongli 7y ago
    I'm still a beginner at Idris, but here's my attempt at a "naive" wc: module Main wordCount : String -> IO () wordCount file = let lineCount = length $ lines file wordCount = length $ words file charCount = length $ unpack file in putStrLn $ "Lines: " ++ show lineCount ++ " words: " ++ show wordCount ++ " chars: " ++ show charCount main : IO () main = do (_ :: filename :: []) <- getArgs | _ => putStrLn "Usage: ./wc filename" (Right file) <- readFile filename | (Left err) => printLn err wordCount file It counts lines, words, and characters. It reports errors such as an incorrect number of arguments as well as any error where the file cannot be read. Here are the stats it reports on its own source code: $ ./wc wc.idr Lines: 14 words: 86 chars: 581 Hope that helps.
    - zeptomu 7y ago
      Wow, thanks for the response - that actually looks like Haskell and if its performance is not that abysmal (let's say slower than Python), that is pretty cool. This actually looks quite reasonable - usage string included, I like it. Hats off to you, I will try to compile it to a binary now and do some benchmarking.
      - seanmcdirmid 7y ago
        Is it really fair to call this an implementation of wc if it is just using a built in function for each case?

seanmcdirmid 7y ago

I don’t know about Idris, but the basic idea would be transform the file’s type from (a|b)* into (a* b)* a* where b is a newline and a is any non newline. Then your answer simply counts the first star of the transformed type (change that star to a counter exponent as you are forming the type).

zeptomu 7y ago
Yeah ... but how does that look? A "naive" wc implementation (counting the character '\n') is <50 lines of code (if we have modern file IO in the standard library) in a practical PL. But maybe there is a too big difference between the concept of "files" (as seen by OS developers that view them as a byte-blob in a hierarchical name system) and language designers that want to represent a file as a collection of lines (and probably would like your command line interpreter not allow to call "wc" on a file that contains non UTF8 encoded data) - but this is still a pretty big step for our current computation environment (where I can call programs by name that are also files). For many people (including me) a "practical" programming language needs an implementation that can generate (compile) an executable or interpret a file representation of its source. Otherwise its a proof assistant. Maybe we have to merge the concept of data in terms (no pun indented) of bytes (types?) in-memory and collections of bits and bytes that have a lifetime longer than the program (if it's not a daemon)?
- seanmcdirmid 7y ago
  If you implement by manipulating the types directly, a few operations with a loop (just for counting lines mind you). Again, I’m not sure how this would be done in Agda, Idris, or COQ, and the system designing is a bit different from other type systems. But also, any discrete one dimensional problem like wc is going to benefit from a type system with types that resembles regular expressions.
- chongli 7y ago
  I'm still a beginner at Idris, but here's my attempt at a "naive" wc: module Main wordCount : String -> IO () wordCount file = let lineCount = length $ lines file wordCount = length $ words file charCount = length $ unpack file in putStrLn $ "Lines: " ++ show lineCount ++ " words: " ++ show wordCount ++ " chars: " ++ show charCount main : IO () main = do (_ :: filename :: []) <- getArgs | _ => putStrLn "Usage: ./wc filename" (Right file) <- readFile filename | (Left err) => printLn err wordCount file It counts lines, words, and characters. It reports errors such as an incorrect number of arguments as well as any error where the file cannot be read. Here are the stats it reports on its own source code: $ ./wc wc.idr Lines: 14 words: 86 chars: 581 Hope that helps.
  - zeptomu 7y ago
    Wow, thanks for the response - that actually looks like Haskell and if its performance is not that abysmal (let's say slower than Python), that is pretty cool. This actually looks quite reasonable - usage string included, I like it. Hats off to you, I will try to compile it to a binary now and do some benchmarking.
    - seanmcdirmid 7y ago
      Is it really fair to call this an implementation of wc if it is just using a built in function for each case?