I get an error in ghci when I try to define a new type:
Prelude> data Point = Pt Int Int <interactive>:1:0: parse error on input `data' Prelude> let data Point = Pt Int Int <interactive>:1:4: parse error on input `data'
What am I doing wrong?
I get an error in ghci when I try to define a new type:
Prelude> data Point = Pt Int Int <interactive>:1:0: parse error on input `data' Prelude> let data Point = Pt Int Int <interactive>:1:4: parse error on input `data'
What am I doing wrong?
ghci does not allow you to define types from interactive input - instead, you need to put your type definition in a file and :load
the file into ghci.
Just for historical reference, the HBI Haskell interactive environment allows for full Haskell at the command line, including types, classes and so on. There's no a priori GHCi can't operate similarly, and users could write a front-end to GHC-API that supported this...
titaniumdecoy
, I remember being helped with this sort of GHCi mystery when I learned the frequently made point that writing things like 'let square x = x * x' inside the GHCi is like writing let a = f b
with do
notation in the IO
monad -- say in this sort of example:
palindromize :: IO ()
palindromize = do
a <- readFile "foo.txt"
let b = reverse a
putStrLn (a ++ b)
Similarly, when you redefine an expression in the GHCi, it's sort of like doing the following in do
notation, which is perfectly legitimate:
makeMess :: IO ()
makeMess = do
a <- readFile "foo.txt"
let b = reverse a
putStrLn (a ++ b)
let b = a
putStrLn (a ++ b)
No one would declare a data type in the middle of such a sequence, but would do it elsewhere in the module. I might have guessed that there was some sort of theoretical objection, but Don S.'s remark suggests there isn't one.