Nim

Other Nim solutions.

import sets, unicode, sequtils, sugar

proc isIsogram*(str: string): bool =
  var seen: HashSet[Rune] = initHashSet[Rune]()
  for c in toLower(str).toRunes.filter(c => c.isAlpha):
    if seen.containsOrIncl(c):
      return false
  return true

Elm

Other Elm solutions.

module Isogram exposing (isIsogram)

import Set


isIsogram : String -> Bool
isIsogram sentence =
    let
        letters =
            sentence
                |> String.toList
                |> List.map Char.toLower
                |> List.filter Char.isAlpha
    in
    List.length letters == Set.size (Set.fromList letters)

Roc

Other Roc solutions.

module [isIsogram]

isIsogram : Str -> Bool
isIsogram = \phrase ->
    sanitized =
        phrase
        |> Str.toUtf8
        |> List.map toLowerCase
        |> List.keepIf isAlpha
    List.len sanitized == Set.fromList sanitized |> Set.len

isAlpha = \char ->
    lower = toLowerCase char
    lower >= 'a' && lower <= 'z'

toLowerCase = \char ->
    when char is
        _ if char >= 'A' && char <= 'Z' -> char + 32
        _ -> char