Curry: An Integrated Functional Logic Language

10 Literate Programming
To encourage the use of comments for the documentation of programs, Curry supports a literate
programming style (inspired by Donald Knuth’s “literate programming”) where comments are the
default case. Non-comment lines containing program code must start with “>” followed by a blank.
Using this style, we can define the concatenation and reverse function on lists by the following
literate program:
The following function defines the concatenation of two lists.
Note that this function is predefined as ‘++’ in the standard prelude.
> append :: [t] -> [t] -> [t]
> append [] x = x
> append (x:xs) ys = x : append xs ys
As a second example for list operations, we define a
function to reverse the order of elements in a list:
> rev :: [t] -> [t]
> rev [] = []
> rev (x:xs) = append (rev xs) [x]
To distinguish literature from non-literate programs, literate programs must be stored in files
with the extension “.lcurry” whereas non-literate programs are stored in files with the extension
“.curry”.
11 Interactive Programming Environment
In order to support different implementations with a comparable user interface, the following commands
should be supported by each interactive programming environment for Curry (these commands
can also be abbreviated to :c where c is the first character of the command):
:load file Load the program stored in file.curry.
:reload Repeat the last load command.
expr Evaluate the expression expr and show all computed results. Since an expression could
be evaluated to a disjunctive expression (cf. Section 3), the expression could be automatically
wrapped in some search operator like depth-first search or a fair breadth-first search,
depending on the implementation.
:debug expr Debug the evaluation of the expression expr, i.e., show the single computation steps
and ask the user what to do after each single step (like proceed, abort, etc.).
:type expr Show the type of the expression expr.
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