Hi,
When you create a record in haskell, it automatically generates
gettor functions for accessing the record fields. It also provides
you with a record update syntax:
rec {field=val}
but there is no settor function. It would be nice if there was some
sort of section-like syntax to access the settor function, like:
{field=} -- val rec
{field=val} -- rec
or even if there was a name for that function (such as field_settor).
Without such a syntax, one often has to write lambdas like:
\r v -> r {field=v}
\r -> r {field=3}
Consider for example an cmd line parser that collects flags in
a record. Instead of writing:
opt "l" (NoArg {c_loop=False}) "dont loop",
opt "o" (optStr {c_out=} "dir") "output dir",
opt "t" (optInt {c_maxtime=} "mins") "maximum time"
we would have to write it out as:
options = [
opt "l" (NoArg (\c -> c {c_loop=False})) "dont loop",
opt "o" (optStr (\v c -> c {c_out=v}) "dir") "output dir",
opt "t" (optInt (\v c -> c {c_maxtime=v}) "mins") "maximum time"
] where
opt cs d s = Option cs [] d s
optStr f name = ReqArg f name
optInt f name = ReqArg (f . read) name
getOptions :: String -> [String] -> IO Config
getOptions prog argv =
let header = "Usage: " ++ prog ++ " [OPTIONS...] files..." in
let usage s = ioError (userError (s ++ "\n" ++ usageInfo header
options)) in
let (updates, files, errs) = getOpt2 options argv in
let config = updates defConfig {c_files=files} in
case () of
_ | not (null errs) -> usage (concat errs)
| null (c_files config) -> usage "no files specified"
| otherwise -> return config
Tim Newsham
http://www.thenewsh.com/~newsham/
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