Thanks everyone. The first suggestion did the trick without needing any other modules.

> What instance of Monoid is this?

I dunno, but the context is a gantt chart drawing program:

data Task = Task { name :: String, desc :: String, dur :: Days } -- tasks is a list of these and deps is a list of...

data Dep = Dep { pre :: String, post :: String } -- referring to Task's name

-- draw the squiggles representing sequential dependencies:

beziset = foldl atop mempty $ map ((maybe mempty id).bezis) deps

bezis (Dep bef aft) =

findIndex (\t-> name t == bef) tasks >>= \bti ->

findIndex (\t-> name t == aft) tasks >>= \ati ->

let t1 = fromIntegral bti in

let d1 = finish (tasks !! bti) in --finish is begin + duration

let t2 = fromIntegral ati in

let d2 = begin (tasks !! ati) in --begin is the latest of the end dates of the directly preceding tasks (or project kickoff)

return ( -- the following monstrosity is just about my coordinate system

fromSegments [bezier3 (r2 (1.3,0)) (r2 (d2-d1-1,-(t2-t1)*(1+gap))) (r2 (d2-d1,-(t2-t1)*(1+gap))) ] #

translate (r2 (descspace+d1,-t1*(1+gap))) )

beziset then gets `atop`ed onto a Diagram.

bezis looks far too long as well. Any way to tidy it up? (For some perverse reason I still don't like do notation.) (I know the bezier3 expression is hideous but I can fix that myself.)

Adrian.

PS: In my previous job I once spent a week evaluating gantt chart drawing softwares. Now I wrote the one I was looking for in half a day. That's Haskell!

On 14 May 2013 20:08, Kim-Ee Yeoh <ky3@atamo.com> wrote:

On Tue, May 14, 2013 at 4:21 PM, Adrian May <adrian.alexander.may@gmail.com> wrote:

I have a really annoying scrap of code:

unmaybe Nothing = mempty
unmaybe (Just dia) = dia

It happened because I'm using Diagrams but building my diagram requires looking something up in a list using findIndex, which returns Maybe Int.

What instance of Monoid is this? Because Int has both a Sum Int and a Product Int instance so you can't just apply unmaybe to (Just 3 :: Maybe Int).

Defining unmaybe Nothing = 0 prompts the question: how will you distinguish misses versus hits on the head of the list? Presumably you don't want to.

You might be interested in the totalized lookup functions defined in my private toolkit (hayoo returns nothing):

-- tlookup :: (Eq a) => b -> a -> [(a, b)] -> b
tlookup b a abs = fromMaybe b $ lookup a abs
tlookup0 a abs = tlookup mempty a abs

-- Kim-Ee

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