Haskell: parse error on input `='
I’ve been trying to follow the ‘Monads for Java/C++ programmers’ post in ghci and getting the following type of error when trying out the code snippets:
Prelude> a = 3 <interactive>:1:2: parse error on input `='
I figured there must be something wrong with my installation of the compiler since I was copying and pasting the example across and having this problem. Having reinstalled that, however, I still had the same problem.
I eventually came across this blog post which points to a mailing list thread from a few years ago where pjd explains that the ‘let’ construct is required when defining a variable from ghci and wouldn’t necessarily be needed in a normal program:
pjd osfameron: about the ghci thing, you have to prefix definitions with "let" as in: let simple x y z = x * (y + z) pjd the reason for this is that ghci is in an implicit do block pjd so it's not exactly like top-level haskell
We have to use a ‘let’ in front of any variable/function definitions and then it will work as expected:
Prelude> let a = 3 3
According to Real World Haskell:
This syntax is ghci-specific The syntax for let that ghci accepts is not the same as we would use at the “top level” of a normal Haskell program.