Re: [Haskell-cafe] beginners question about fromMaybe

Luke's answer is great (although it changes argument order). Hint: http://www.haskell.org/haskellwiki/Things_to_avoid#Avoid_explicit_recursion I also like the "pattern guards" GHC extension; I tend to use it over "maybe" and "either". I find the resulting code more readable:

{-# LANGUAGE PatternGuards #-}

probePhase is sc xs m = concatMap prefix xs where prefix x | Just val <- Map.lookup (getPartialTuple is x) m = joinTuples sc x val | otherwise = []

Alternatively, I might write it like this:

import Control.Monad

maybeM :: MonadPlus m => Maybe a -> m a maybeM = maybe mzero return

probePhase is sc xs m = do x <- xs val <- maybeM $ Map.lookup (getPartialTuple is x) m joinTuples sc x val

This now works for any xs that is an instance of MonadPlus (assuming joinTuples is also polymorphic). Both of these examples are more wordy than Luke's quick two-liner, but, to me, it's worth it for the additional "maintainability" of that code. I am perhaps in the minority on this issue, though :) -- ryan On Tue, Jun 2, 2009 at 4:20 PM, Luke Palmer wrote:

On Tue, Jun 2, 2009 at 4:59 PM, Nico Rolle wrote:

...

hi there

heres a code snipped, don't care about the parameters. the thing is i make a lookup on my map "m" and then branch on that return value

probePhase is sc [] m = [] probePhase is sc (x:xs) m    | val == Nothing  = probePhase is sc xs m    | otherwise       = jr ++ probePhase is sc xs m        where            jr  = joinTuples sc x (fromMaybe [] val)            key = getPartialTuple is x            val = Map.lookup key m

Here's my take.   This ought to be equivalent, but I haven't tested. probePhase is sc m = concatMap prefix     where     prefix x = let key = getPartialTuple is x in                maybe [] (joinTuples sc x) $ Map.lookup key m

...

the line "jr  = joinTuples sc x (fromMaybe [] val)" is kind of ugly because i know that it is not Nothing. is there a better way to solve this? regards _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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