[Haskell-cafe] ReadS with Maybe

Robert Dockins robdockins at fastmail.fm
Mon Jan 3 14:58:39 EST 2005

 From the GHC docs:

type ReadS a = String -> [(a, String)]
A parser for a type a, represented as a function that takes a String and 
returns a list of possible parses (a,String) pairs.

So it returns all possible parses, hence the list.  This is useful if 
the encoding is ambiguous.

Henning Thielemann wrote:

> What is the reason for the definition
>  ReadS a = [(a, String)]
> not being
>  ReadS a = Maybe (a, String)
> ? The latter one reflects that either one or no value is read, whereas the
> first definition allows an arbitrary number of read values which is
> confusing and unsafe in my opinion.
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