jamin1001 wrote:
Hi, I am new at Haskell and have some basic questions.
So then how should this be done? What if I want to do something like
data Chair = Chair {pos:: Int, color :: Int}
data Table = Table {pos:: Int, color :: Int}
Unfortunately you have to think up different names for all constructors and
field names that are declared anywhere in the same module. A possible
methodical way to do this is to prefix all names by the type name (with
first letter lowercase of course) eg:
data Chair = Chair {chair_pos:: Int, chair_color :: Int}
data Table = Table {table_pos:: Int, table_color :: Int}
The reason is that when you declare a fieldname this also introduces a top
level function with the same name which returns the relevant component of
the record ie in the above the following top level functions are generated
by the compiler:
chair_pos :: Char -> Int
chair_color :: Chair -> Int
table_pos :: Table -> Int
table_color :: Table -> Int
Also, could someone tell me why this doesn't compile in GHC:
data Test = A {a::Int} | B {a::Int, b::Int}
data Test2 = C {c::A}
(Test2 line): Not in scope: type constructor or class 'A'
(A) is a data constructor whereas you need a type here, so it should be:
data Test2 = C {c :: Test}
Brian.
--
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