PITFALLS.md

Common Pifalls when Programming in Egel

Egel is a minimalist language, reading the man page should suffice to start programming in Egel.

However, I expect people to encounter the following common pitfalls.

Wrong usage of patterns

The following two patterns are very different. The first defines a function that takes two arguments, the latter defines a function that takes one argument, that one argument being the composite of two terms.

    [X Y -> ... ] [(X Y) -> ... ]

Forget to open namespace in interactie mode

In interactive mode the prelude is loaded and the System and List namespaces are opened. For the other namespace use:

    >> using String;; using Math

No short-circuited boolean operators

The && and || combinators take functions as their second argument. Use in the following manner:

    true || [_ -> false && [_ -> true]]

Passing variadic functions

The following will not have the effect that one would assume it has.

    map print {"hello", 3.14}

Print is a variadic function and Egel has eager semantics. Given that print is variadic it will print nothing and reduce to none after which that will be applied to the constants in the list.

The following is safe, however.

    map [X -> print X] {"hello", 3.14}

Math operators only work on floats

Despite Egel being a dynamic language, the combinators defined in the Math namespace exclusively work on floats. Look in the prelude or cast with to_float and to_int combinators.

Overlapping usings

When two namespaces A and B both define foo and both namespaces are opened, foo will be redeclared and give a runtime error. Handle with care.

    namespace A ( data foo ) namespace B ( data foo ) using A using B