Type-Driven Development with Idris by Edwin Brady

Author:Edwin Brady
Language: eng
Format: epub, pdf
Publisher: Manning Publications

Here, Idris has evaluated 1 + k in the type to S k. But if you try swapping the arguments to +, you’ll get a different result:

*ReverseVec> \k, elem => Vect (k + 1) elem \k => \elem => Vect (plus k 1) elem : Nat -> Type -> Type

Here, Idris has evaluated k + 1 to plus k 1, where plus is the Prelude function that implements addition on Nat. To see the reason for the difference, you need to look at the definition of plus. You can do this using the REPL command :printdef, which prints the pattern-matching definition of the function given as its argument:

*ReverseVec> :printdef plus plus : Nat -> Nat -> Nat plus 0 right = right plus (S left) right = S (plus left right)

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