Standard ML vs Idris
When comparing Standard ML vs Idris, the Slant community recommends Idris for most people. In the question“What are the best languages for learning functional programming?” Idris is ranked 9th while Standard ML is ranked 15th. The most important reason people chose Idris is:
Idris not only has support for type classes, but is a fully dependently typed language, giving you the full power to statically verify your code.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Powerful module system
The module system that Standard ML uses gives the programmer the power to define custom data types whose internal implementation is invisible to other programmers using the module.
Pro Implementing laziness is trivial
Since mutability is only confined to a special type of reference cells, implementing laziness in SML can be done in only 20 lines of code.
Pro Enforces distinction between data and computations
Since it uses strict evaluation, it enforces distinction between data and computations which in turn enables you to use induction on algebraic data types as a reasoning principle.
Pro Great exception system
Secret messages can be sent across distant parts of a program without possibility of being intercepted by unintended recipients in the middle.
Pro Full dependent types
Idris not only has support for type classes, but is a fully dependently typed language, giving you the full power to statically verify your code.
Pro Domain driven design and type driven development
Because of full dependent types in Idris, the programmer can focus more on modelling the domain with types and waste less time fixing common bugs that the type checker will catch. Dependent types help apply type driven development and a lot of code auto generation, making the compiler and type checker an ally in developing working software instead of just getting in the way.
Cons
Con Not very popular outside academia
SML is mostly used in academia and doesn't have many uses in industry. While it's a good language for learning functional programming concepts, the language itself won't be very useful.
Con Not widely used
Con Not widely used
Con Weaker type inference
As type inference is undecidable for dependently-typed languages, Idris cannot offer the full type inference that Haskell supports, and so more type annotations will be needed.
Con Different semantics from Haskell
Idris, while similar to Haskell, has strict semantics, which may cause some confusion if your backend is done in Haskell. If using Idris, it would make sense to do the backend in Idris as well, if not for the fact that Idris currently has fewer libraries available for web development than Haskell.