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Custom Literals

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vorov2 edited this page Feb 26, 2020 · 2 revisions

Introduction

This article discusses custom literals in Ela.

Overview

Ela (starting from version 0.12.7) supports a notation for custom numeric literals. A custom literal is combined from an integer or real literal and a char postfix (e.g. 12p). Custom literals are implemented as functions and processed by parser in the same way as standard numeric literals and have the highest priority among all other operations.

Creating a custom literal

In order to create a custom literal one should write a regular Ela global function which name is combined from the word literal, prime symbols and a literal postfix. For example, this is how a custom literal for peano numbers is implemented:

literal'p s = fromIntegral x
  where x = read s ::: Long

In the example above literal p is a regular Ela function and can be applicated like any other functions. However, one can also invoke this function using a literal notation:

x = 12p //Is equivalent to: x = literal'p 12

When defining a custom literal function a postfix part of its should be always in a lower case, e.g. literal'p is a correct name while literal'P is not and won't be recognized as a literal processor function. However, when specifying a literal postfix it is possible to use both uppercase and lowercase forms. E.g. the following examples are equivalent:

x = 12p
y = 12P

Notes on implementation of literal functions

When Ela sees an occurence of a custom literal expression, such as 12p in the examples above it doesn't parse this expression at all. Therefore a literal function should always accept just a single argument and this argument is always of type String. This string doesn't include a literal postfix, e.g. when somebody defines a number like so: 12p, a literal function literal'p would received a single argument "12" of type String.

The string, provided to the literal function, should be manually parsed inside the function. In a lot of cases it is possible to used standard prelude.read function for that, just like in the example of peano number literal function above.

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