hlint

Back in the early days of Unix, when the PDP--11/70's main advantage over the later 8/16-bit 6809 was having separate I/D (instruction and data) space, so that you could have 64K of code with access to 64K of data, the virtue of the Unix Way of small programs that did one thing and did it well was a necessity. One of the things it gave rise to was a separate program, "lint", to check C source code for constructs that might be evidence of a coding error, so that the compiler could concentrate on simply generating code.

Nowadays, C compilers often do some of the checking that was once delegated to lint (though separate lint programs still exist, and are very good--e.g. splint, or Gimpel Software's excellent products).

What about Haskell? Well, for Haskell there's hlint. It will give you advice on the Haskell source that you feed it.

jejones@eeyore:~/src/haskell_play$ hlint ultimatepalindrome14.hs
ultimatepalindrome14.hs:236:13: Warning: Use zipWith
Found:
  map showsResult $ zip [1 ..] (map process (tail $ B.lines s))
Why not:
  zipWith (curry showsResult) [1 ..] (map process (tail $ B.lines s))

ultimatepalindrome14.hs:237:5: Error: Use .
Found:
  mapM putStr $ map ($ "\n") r
Why not:
  mapM (putStr . ($ "\n")) r

2 suggestions
jejones@eeyore:~/src/haskell_play$


So there are (at least!) a couple of places I could have written arguably better, easier to read, more idiomatic Haskell. Pretty cool.

What's with that "."? In that context, "." is the function composition operator; given two functions, one of which returns values of the type the other takes as input, composing them gives you a function that applies first one function, then the other. Huh? Better to write it in Haskell:

. :: (a -> b) -> (c -> a) -> (c -> b)

f . g  x = f (g x)

or, equivalently,

f . g = \x -> f (g x)

\ isn't a character escape; it's the closest the creators of Haskell could come to  the Greek letter lambda, as in Alonzo Church's "lambda calculus". A lambda expression is an "anonymous function"; the way to read the right hand side of that last line is "the function that, given an argument x, returns f (g x)".

(Ironically, the lambda was in turn a sort of best approximation; it was inspired by the use of the circumflex in the notation of Russell and Whitehead's Principia Mathematica. Details here.)

Anyway... there's also a Haskell "style scanner" that one can either just get suggestions from or use in the fashion of indent. I will have to check it out.

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