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We have a guide to acceptable input and output methods on Meta, and this currently only specifies that input may be taken by direct insertion into the source code for languages that have no other option.

For languages that can take input, it can be taken by any of STDIN, command line arguments, or function arguments. For example, in Python this is often as the argument to a lambda function.

Currently this answer will not be valid until n is taken as an input, but if you think the rule should be changed then anyone is free to post a new rule under that Meta post, to see if the community votes to support it.

I've changed both. I made lambda functions from both expressions. Is this enough?

I haven't tested the Haskell code, but there's a problem with the Python code that I'll explain below.

In Python, a lambda function cannot have multiple statements. If you try to run the current code, it will give an error. I can think of 2 different approaches to avoid this problem:

• Use a lambda function with only a single statement
• Use a def function to allow multiple statements

I'll put examples of these 2 approaches in the next 2 comments for comparison.

lambda n:(4*("IVXLCDM".index(n)%2==1)+1)*10**("IVXLCDM".index(n)//2)

This can be tested as follows:

f=lambda n:(4*("IVXLCDM".index(n)%2==1)+1)*10**("IVXLCDM".index(n)//2)

for c in "IVXLCDM":
    print(c, f(c))

def f(n):i="IVXLCDM".index(n);return(4*(i%2==1)+1)*10**(i//2)

This can be tested as follows:

def f(n):i="IVXLCDM".index(n);return(4*(i%2==1)+1)*10**(i//2)

for c in "IVXLCDM":
    print(c, f(c))    

To summarise the difference between def and lambda in Python, the following give equivalent results:

def double(n):
    return n * 2

double = lambda n: n * 2

The problem is fixed, and thanks to the walrus operator I can still use the lambda function. And it is even shorter than the wrong version.