I was writing some Haskell code where I was constructing a value with lots of record fields, but only cared about one of them initially.

data Type = Type { field1 :: Type1, field2 :: Type2 }

But, omitting the irrelevant fields from the initializer causes a big "Fields not initialised" warning.

So, I figured that, instead of writing out

Type { field1 = value, field2 = undefined }

which was verbose, or

Type { field1 = value }

which resulted in the big warning, I could use

undefined { field1 = value }

which did eliminate the warning.

However,

field1 (undefined { field1 = value })

resulted in an exception. I had expected it to be equivalent to

field1 (Type { field1 = value, field2 = field2 undefined })

but it's not. Since types can have multiple constructors, it's actually equivalent to

field1 (case undefined of { Type { field2 = field2 } -> Type { field1 = value, field2 = field2 })

according to section 3.16.3 in the Haskell report.

While trying to write concurrent code that forks off threads that sleep before doing some delayed actions using GHCi, version 6.10.4, which is a pretty old version, I found some strange behavior. I used map (flip addUTCTime time) [1..5] to generate the times that the delayed actions should be performed, and the interpreter would lock up. When I changed it to map (flip addUTCTime time) [1,2,3,4,5], everything worked as expected. Maybe there is something tricky in the implementation of enumFromTo :: NominalDiffTime -> NominalDiffTime -> [NominalDiffTime]