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provides ordering by precedence for type-check
LiveScript JavaScript
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type-precedence Build Status

type-precedence is a library used to determine precedence of types that can be checked with type-check. So, if a value mathes more types, you can find the most specific one. Used, for example in defn, to choose the best signature of an overloaded function.

For reference on how a type can be specfied, please see Type Format or Quick Examples for type-check

$ npm install type-precedence

Type Precedence

  • __ <= * - * is the most general type (`_` stands for any type)_
  • {x: __} < Object - an explicit object is more specific than Object
  • [__] < Array - same as for object
  • {x: __} < {...} - a subset is more general
  • {x: __, y: __, ...} < {x: __, ...} - a subset with more keys specified is more specific
  • TypeA < TypeA | TypeB
  • TypeA < Maybe TypeA
  • Number | String < String | Number for value 1
  • String | Number < Number | String for value 's'
  • everything* above applied recursively for [arrays], {fields} or (tuples) (e.g {x: [Number]} < {x: [*]})

*actually almost everything - (String | Number, *) and (Number | String, *) would be equal for any value. So ambiguous | (i.e. where you need a target value to decide which is best) will not work recursively.


the following examples are written in LiveScript, but, of course, the library can be used for javascript too.

{best-type, sort-types, compare-types} = require \type-precedence

best-type in: types[, matching: target]

Gets the most specific type from a list of types, optionally matching a target value

best-type in: <[Array [*] *]> # [*]
best-type in: ['Number | String', 'String | Number'], matching: 1 # 'Number | String', since the value is a Number

sort-types types[, matching: target]

Sorts the types by precedence - the most specific will be first in the sorted list. It optionally checks against a target value

sort-types <[Array [*] *]> # <[[*] Array *]>
sort-types ['{x: Number, ...}' '{...}' '{x: *}' '*' 'Object'] # ['{x: *}' '{x: Number, ...}' '{...}' 'Object' '*']

compare-types type-a, type-b[, matching: target]

Compare two types, optionally checking against a target value. It returns -1, 0, or 1.

compare-types '{x: String, ...}', '{x: *, ...}' # -1
compare-types '(*)', '(Object)' # 1
compare-types '{x: *, ...}', '{y: *, ...}' # 0 - two subsets with same number of specified keys

... matching: target

Providing the optional argument matching: target to any of the functions above will do the following:

  • help choosing the best type when ambiguous
compare-types 'Number | String', 'String | Number' # 0, because there is no clear winner
compare-types 'Number | String', 'String | Number', matching: \string # 1, because 'String | Number' is more specific in this case
  • validate the types against the target value, throwing an Error if one of the types doesn't match
compare-types 'Number', 'String' # 0
compare-types 'Number', 'String', matching: 1 # throws '<1> does not match <String>'
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