Am Sonntag 30 August 2009 14:32:56 schrieb Patrick LeBoutillier:
Hi,
volume :: Fruit FruitType -> Double
volume F{radius=r,len=l,fType=Orange} = (4.0/3.0) * pi * r * r * r
volume F{radius=r,len=l,fType=Apple} = (4.0/3.0) * pi * r * r * r volume F{radius=r,len=l,fType=Banana} = pi * (r * r) * l volume F{radius=r,len=l,fType=Watermelon} = (4.0/3.0) * pi * (2.0 * r) * l * (0.5 * l)
Can anyone explain the above pattern matching syntax? I've never seen it before...
Thanks,
Patrick
It's named-field syntax, cf. http://haskell.org/onlinereport/exps.html#sect3.15 If you have a type with named fields, like data FType = Con1 { field1 :: Int, field2 :: Bool } | Con2 { field1 :: Int, field3 :: Char } you can pattern-match either by position: fun (Con1 i b) = ... or by named field syntax: fun Con1{field2=True, field1=x} = ... -- corresponds to fun (Con1 x True) fun Con2{field3='a'} = ... -- fun (Con2 _ 'a') It's very convenient if you need only a few arguments of a multi-argument constructor: fun C{fieldx=y} vs. fun (C _ _ _ _ _ y _ _ _ _)