Instances of Read and Show
Thu, 29 Mar 2001 17:10:10 +0200 (MET DST)
I have a question concerning the Read and Show classes.
I have read the Haskell 98 report (section 6.3.3 The Read and Show
Classes), but I still do not figure out how to make instances.
I have a simple data type:
data TVal a = Abst
| Prst a
I managed to define the function show, so that I can use it with the data
instance (Show a) => Show (TVal a) where
show Abst = "_"
show (Prst x) = show x
Signals> [Prst 1, Abst, Prst 2]
[1,_,2] :: [TVal Integer]
which is what I want to have.
But how do I make a Read instance?
I still do not understand the meaning of 'readsPrec' and
'readList' (and not of 'showPrec' and 'showList') either.
Can somebody explain to me or give me a good reference so that I can make
TVal an instance of the classes Read and Show.
Thanks in advance for your help!